[llvm] workflows/pr-subscriber: Use our own custom concurrency implementation (PR #66263)

[Tobias Hieta via llvm-commits]

https://github.com/tru commented:

Not a super fan of having the python script in the action yaml. Can't we have that checked in instead? I understand we need to fetch it, but it seems more maintainable. 

https://github.com/llvm/llvm-project/pull/66263