[llvm] workflows: Unsplit new-prs (PR #69560)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 approved this pull request.

LGTM. This definitely makes things look quite a bit cleaner.

>From what I understand, it's not that easy to misuse `pull_request_target` as you have to explicitly run the checkout action with a different ref.

Should probably be safe, but take that with a grain of salt as I am definitely not a CI/CD security expert.

https://github.com/llvm/llvm-project/pull/69560