[llvm] [Github] Make PR formatting job only run with C/C++ changes (PR #69556)

[Tom Stellard via llvm-commits]

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

This job needs to be re-written to not use pull_request_target, but that's unrelated to this change. LGTM.

https://github.com/llvm/llvm-project/pull/69556