Add bazel mirror to BCR config

[rickeylev]

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.