Add a comment if the CI fails on a PR that is already closed

We currently have some PRs being mirrored from internal VCS to Github. If a job on such a PR fails that was opened by graalbot, we should probably make sure to ping the Oracle employee who opened the internal PR, e.g. using https://github.com/actions/github-script#comment-on-an-issue - we could do it only if the author is graalvmbot and there's just a single assignee.