github: Fix automated PR creation for backports

github: Fix automated PR creation for backports

GitHub Actions stores the token used for checking out a git repo in
the git configuration and then uses that token for pushes from that
repo too.  We need to use a different token for push because we
are pushing to the llvmbot/llvm-project repo and not the upstream repo,
so we need to disable persist-credentials when checking out the source.