[ci] Temporary fork runtime_commit_artifacts

Unfortunately creating a workflow that depends on another worfklow run
requires it to first be merged into main, so I can't really test porting
this without landing it first.

To do this safely, I've left the original job intact for DiffTrain and
added a forked file. This fork only currently downloads the artifact
from the HEAD commit in GH actions; I've removed the steps that push to
the protected branches for now while I test to see if this works as
expected.

This workflow needs to depend on the runtime_build_and_test workflow
being complete because otherwise it will fail since the artifacts
haven't been built yet.

ghstack-source-id: 0f9cebc5251ee8339ca95e2d658d7e75fc2a213e
Pull Request resolved: https://github.com/facebook/react/pull/30472