[llvm] [GitHub][workflows] Add buildbot information comment to first merged PR from a new contributor (PR #78292)

[Alex Bradbury via llvm-commits]

asb wrote:

This is a great idea - thanks! A few thoughts:
* Are we certain that buildbot emails do go to the patch author and not just the committer? CC @gkistanova.
* "If you are unsure how to fix a problem, you can send questions in a reply to the notification email". If your change is the only one causing a failure, then the only person to reply to is gkistanova. Otherwise, you're replying to other patch authors in the change list. Perhaps directing people to the PR or to discord would be better?
* We do seem to have some buildbots that have spurious failures every now and again. I think it would be good to warn of that possibility. e.g. "On occasion buildbots will encounter a spurious failure (i.e. failing for a reason other than your change). If after reviewing the details of the failure you suspect this is the case, you may wish to check if there were similar failures in recent builds on that bot or wait to see if the next scheduled build fails." Maybe that's getting a bit lengthy though...

https://github.com/llvm/llvm-project/pull/78292