mac: Building with --cpu=darwin_arm64 generates x86_64 binaries

Building on MacOS 14.7 (Apple M1) with Xcode 16.2.

Steps to reproduce:

1. Build main branch with `bazelisk build --config=goma //:artifacts_tar --cpu=darwin_arm64`
2. Check the resulting binary architecture:

```
% file /private/var/tmp/_bazel_marshall/15a38429f9b5a1eebde75709bdec5b6b/execroot/_main/bazel-out/darwin_arm64-opt/bin/artifacts/rewrapper
/private/var/tmp/_bazel_marshall/15a38429f9b5a1eebde75709bdec5b6b/execroot/_main/bazel-out/darwin_arm64-opt/bin/artifacts/rewrapper: Mach-O 64-bit executable x86_64
```

Actual vs expected result:

Resulting binaries should be arm64 instead of x86_64.

Additional details:

Adding the `--macos_cpus=arm64` flag doesn't help.

Switching between x86_64 and arm64 bazelisk/bazel binaries (installed via Homebrew) doesn't help.

Reproduce the report on macOS 14.7 with `bazelisk build --config=goma //:artifacts_tar --cpu=darwin_arm64`, then inspect the generated `artifacts/rewrapper` with `file`. Trace the build configuration that selects the binary architecture; done means the resulting binaries report arm64 rather than x86_64, including when `--macos_cpus=arm64` is omitted.

Written by the indexing model from the issue text.