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

[David Spickett via llvm-commits]

DavidSpickett wrote:

Important to note that this comment will notify all users, there's no way to avoid that. I hope that by `@` ing the author, it may be more friendly to email filters.

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