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

[Tom Stellard via llvm-commits]

tstellar 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.

This patch enables doing this too.  We already had manual dispatch, but it only supported tags, with this PR, you can feed it an arbitrary commit.

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