404s for last_green bazel

Not sure which repo this belongs on, but it affects CI

https://buildkite.com/bazel/rules-swift-swift/builds/2892#f81e439f-881f-4b1e-b0cb-733f6a85928d

```
% USE_BAZEL_VERSION=last_green bazelisk version
2021/10/26 14:05:23 Using unreleased version at commit c37bd0d027629bf0e3418208bb5ef9f6c4507f54
2021/10/26 14:05:23 Downloading https://storage.googleapis.com/bazel-builds/artifacts/macos/c37bd0d027629bf0e3418208bb5ef9f6c4507f54/bazel...
2021/10/26 14:05:23 could not download Bazel: HTTP GET https://storage.googleapis.com/bazel-builds/artifacts/macos/c37bd0d027629bf0e3418208bb5ef9f6c4507f54/bazel failed with error 404
```

Start with the linked Buildkite failure and reproduce `USE_BAZEL_VERSION=last_green bazelisk version` from the issue. Trace why the selected commit's macOS artifact returns HTTP 404; done means the last_green artifact can be downloaded successfully in CI.

Written by the indexing model from the issue text.