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

[David Spickett via llvm-commits]

DavidSpickett wrote:

> "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 (I think [llvm.buildmaster at lab.llvm.org](mailto:llvm.buildmaster at lab.llvm.org) just bounces). Otherwise, you're replying to other patch authors in the change list. Perhaps directing people to the PR or to discord would be better?

Yeah, not mentioning the email reply is probably better. The reviewers of the PR are more likely to have context on why it failed (I would hope), and they can sort it out from there.

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