Do not set `RUSTFLAGS` environment variable when invoking Cargo commands via Gradle tasks (#3678)

Otherwise, if you generate a crate and compile it using Gradle, like for
example using the invocation:

```
./gradlew -P modules='simple' -P cargoCommands='test' codegen-server-test:build
```

And then manually run a `cargo` command within the generated crate
directory, or open the project using `rust-analyzer`, Cargo will
re-compile the project from scratch, with
`CARGO_LOG=cargo::core::compiler::fingerprint=trace` reporting that
flags have changed since the last compilation.

Instead, it's best if we persist these flags to `.cargo/config.toml`, so
all `cargo` invocations, either through Gradle, manually, or through
`rust-analyzer`, use the same set. This way, if no files were changed,
subsequent compilations since code generation will truly be no-ops, with
Cargo reusing all artifacts.

Note this commit fixes a regression that was introduced when `--cfg
aws_sdk_unstable` was introduced in #2614, since I fixed this the first
time back in #1422.

----

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