[llvm] workflows: Add a new job for packaging release sources (PR #91834)

[Tobias Hieta via llvm-commits]

tru wrote:

If I understand this correctly - this step will replace the export command I run locally to create the source packages? Instead I will download the sources from this WF, PGP sign it and then upload to the release?

https://github.com/llvm/llvm-project/pull/91834