set a PR's milestone based on its merge branch

If there's an open milestone with description "Merge to ${pull.base.ref}.",
set it as the PR's milestone.
avoid false positives due to jobs racing

when a faster job completes successfully before a
longer-running job has a chance to indicate failure,
we may get a false positive

we fix this by prepending the pending status/failed of other jobs
that have a pending/failed status but no error/success result