Add bazel mirror to BCR config

The workspace config shows a bazel mirror, e.g.

```
https://mirror.bazel.build/github.com/bazelbuild/platforms/releases/download/1.1.0/platforms-1.1.0.tar.gz
```

However, the BCR config doesn't have this mirror listed: https://github.com/bazelbuild/bazel-central-registry/blob/main/modules/platforms/1.1.0/source.json

Given that Github requests regularly give errors, having the mirror listed in the BCR config would help reduce flakes in CI.

Compare the workspace config's bazel mirror with the BCR entry at modules/platforms/1.1.0/source.json, starting with the existing source URLs. Add the mirror entry for the platforms 1.1.0 archive, then verify that the BCR configuration contains both the GitHub and mirror URLs.

Written by the indexing model from the issue text.