[llvm] workflows/release-binaries: Add schedule to run job once per month (PR #73812)

[Tobias Hieta via llvm-commits]

tru wrote:

Ah this is nice. I think we should be able to manually dispatch the workflow from main as well, because we might want to trigger it a little before we branch so that we are sure it works.

https://github.com/llvm/llvm-project/pull/73812