Run `semver-checks` on PRs submitted by external contributors (#3291)

## Motivation and Context
Enables `cargo semver-checks` in CI for PRs created by external
contributors

## Description
For instance, we skipped a run of `cargo semver-checks` in
https://github.com/smithy-lang/smithy-rs/pull/3286 and failed to detect
[a breaking
change](https://github.com/smithy-lang/smithy-rs/pull/3286#discussion_r1416479632)
programmatically.

With this PR, the workflow will run a job `semver-checks` even if the
preceding jobs `save-docker-login-token` or `acquire-base-image` are
skipped. Those jobs are relevant when the PR made changes to build
tools, which is less likely for PRs created by external contributors, so
it's reasonable to skip them and still run the `semver-checks` job.

Furthermore, this PR enables `semver-checks` to run against all crates
in `tmp-codegen-diff/aws-sdk/sdk/`, not just those limited by
`list(os.listdir())[:10]`.

## Testing
Tested the change against [a dummy
PR](https://github.com/smithy-lang/smithy-rs/pull/3288) I created from
my fork of `smithy-rs`. Specifically, `semver-checks` [caught the
aforementioned breaking
change](https://github.com/smithy-lang/smithy-rs/actions/runs/7121830175/job/19391798131#step:4:681)
in CI.

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