Upload shard distribution per platform

[fweikert]

This change allows us to better understand and benchmark the sharding strategy.

Tested:

• https://buildkite.com/bazel/bazel-fwe-ms-test/builds/24#01924d33-2fa7-4a90-b518-6d95431fdfd2
• https://storage.googleapis.com/bazel-untrusted-buildkite-artifacts/01924d33-2fa7-4a90-b518-6d95431fdfd2/tmp/tmp4nl2p4v3/shard_distribution.json