[EngSys] Extract PR number from check_suite:completed event from fork PR (if possible)

[mikeharder]

In a fork PR, the pull_requests property in the payload will be empty for fork PRs. For workflow_run:completed, it's possible to extract the PR number from the payload, by calling an API in the head (fork) repo.

So far, I haven't figured out how to do the same for check_suite:completed on fork PRs. This is needed for checks like "ARM Auto Signoff (Preview)".

The check_suite:completed event provides the head_sha, but not the owner of the fork repo containing the SHA.

[mikeharder]

Appears to be impossible with the information in check_suite:completed from fork PRs. Workaround was to trigger on issue_comment:edited if body contains "next steps to merge" (#32554)