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

[David Spickett via llvm-commits]

DavidSpickett wrote:

https://discourse.llvm.org/t/rfc-buildbot-information-comment-on-new-contributors-first-prs/76303 to get more diverse feedback.

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