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

[Alex Bradbury via llvm-commits]

https://github.com/asb approved this pull request.

LGTM. I'd be OK with just having the comment in unwrapped form so that it's stored in the preferred form to be submitted as a GitHub comment, but the trade-off you make seems reasonable too.

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