Fix running semver-checks on `aws-config` (#3272)

## Motivation and Context
Fixes https://github.com/smithy-lang/smithy-rs/issues/3265

## Description
To check semver, `semver checks` needs to compile two versions of a
crate, i.e. baseline and current. Our CI failed to compile `aws-config`
for a baseline version.

The following diagram shows what was missing to cause compilation
failure:
```
┌───────────┐
│ smithy-rs │
└─────┬─────┘
      │
      │
      │     ┌────────┐
      └─────┤ target │
            └───┬────┘
                │        ┌───────────────┐
                └────────┤ semver_checks │
                         └───────┬───────┘
                                 │
                                 │
                                 │       ┌──────────┐
                                 └───────┤ git-base │
                                         └────┬─────┘
                                              │
                                              │        ┌──────────────┐
                                              ├────────┤ <rev number> │
                                              │        └──────┬───────┘
                                              │               │
                                              │               │
                                              │               │       ┌───────┐
                                              │               ├───────┤  aws  │
                                              │               │       └───┬───┘
                                              │               │           │
                                              │               │           │       ┌──────────────┐
                                              │               │           ├───────┤ rust-runtime │
                                              │               │           │       └──────┬───────┘
                                              │               │           │              │
                                              │               │           │              │       ┌────────────┐
                                              │               │           │              └───────┤ aws-config │ ◄***************
                                              │               │           │                      └────────────┘                *
                                              │               │           │                            *                       *
                                              │               │           │                            *                       *
                                              │               │           │          *****depends*on****                       *
                                              │               │           │          ▼                                         *
                                              │               │           │                                                    *
                                              │               │           │       ┌─────┐                                      *
                                              │               │           └───────┤ sdk │ (with no "build" directory)          *
                                              │               │                   └─────┘                                      *
                                              │               │                                                                *
                                              │               │                                                                *
                                              │               │      ┌────────────────────┐                                depends on
                                              │               └──────┤  tmp-codegen-diff  │                                    *
                                              │                      └─────────┬──────────┘                                    *
                                              │                                │                                               *
                                              │                                │       ┌─────────┐                             *
                                              │                                └───────┤ aws-sdk │                             *
                                              │                                        └────┬────┘                             *
                                              │                                             │     ┌─────┐                      *
                                              │                                             └─────┤ sdk │                      *
                                              │                                                   └─────┘                      *
                                              │                                                                                *
                                              │                                                                                *
                                              │           ┌───────────────────────────────────────┐                            *
                                              └───────────┤ local-aws_config-0_0_0_smithy_rs_head │*****************************
                                                          └───────────────────────────────────────┘
```
`local-aws_config-0_0_0_smithy_rs_head` under the `git-base` directory
is a special crate created by `semver-checks` for a baseline version of
`aws-config` and its `Cargo.toml` depends on
`target/semver_checks/git-base/<rev
number>/aws/rust-runtime/aws-config`. However, that `aws-config` in turn
depends upon crates in `target/semver_checks/git-base/<rev
number>/aws/sdk/build/` (as shown
[here](https://github.com/smithy-lang/smithy-rs/blob/main/aws/rust-runtime/aws-config/Cargo.toml#L23-L33)),
which does not exist in a baseline branch.

When `semver-checks.py` [creates a baseline
branch](https://github.com/smithy-lang/smithy-rs/blob/3d0cb5c3b179d5ed1d14531882657b481c4469f0/tools/ci-scripts/codegen-diff/semver-checks.py#L31)
to prepare for running `cargo semver-checks`, it [moves aws/sdk/build to
tmp-codegen-diff](https://github.com/smithy-lang/smithy-rs/blob/3d0cb5c3b179d5ed1d14531882657b481c4469f0/tools/ci-scripts/codegen-diff/diff_lib.py#L59),
leaving nothing behind in `aws/sdk/build/`. The fix, therefore, is to
`cp -r aws/sdk/build/aws-sdk` instead of `mv aws/sdk/build/aws-sdk` when
preparing a baseline branch.

The issue is specific to `aws-config`, probably because that's the only
runtime crate that depends on those in `aws/sdk/build`.

## Testing
Verified a clean run for `cargo semver-checks` in [an investigation
branch](https://github.com/smithy-lang/smithy-rs/actions/runs/7035082815/job/19144676499#step:4:1101).

----

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