Update runtime workflow to use HEAD commit (#31850)

This updates the CI workflow for the runtime build and tests to use the
HEAD commit of the PR branch rather than the Fake News merge commit that
the `@actions/checkout` action bafflingly defaults to.

Testing against the merge commit never made sense to me as a behavior
because as soon as someone updates upstream, it's out of date anyway.

It should just match the exact commit that the developer pushed, and the
once that appears in the GitHub UI.