[llvm] [GitHub][workflows] Ask reviewers to merge PRs when author cannot (PR #81142)

[David Spickett via llvm-commits]

DavidSpickett wrote:

CI is failing because it's running the workflow from my fork, but it gets the script from main which doesn't have the new action of course. This is expected.

Which seemed a bit scary to me, but going off of https://github.blog/2023-08-09-four-tips-to-keep-your-github-actions-workflows-secure/ "An important security guarantee that GitHub makes is that workflows triggered on pull_request from forks are run with minimal privileges: no access to secrets and the repository token is read-only. " - the workflow wouldn't have been able to post a comment anyway.

https://github.com/llvm/llvm-project/pull/81142