github: Fix release automation /branch command with new repo

We started using the llvm/llvm-project-release-prs repo for
backport pull requests, but since this repo is not a fork of
llvm/llvm-project it will reject pull requests from other repos. In
order to fix this, when ever someone uses the /branch command to request
a branch be merged into the release branch, we first copy the branch to
the llvm-project-release-prs repo and then create the pull request.

Reviewed By: thieta

Differential Revision: https://reviews.llvm.org/D126940