Allow other branch names than master in GitHub action

GitHub is switching the default branch name from master to main. Currently refs/heads/master is hard-coded. It would be good to have this configurable.

https://github.com/lightspeed/palisade/blob/4eef8be1eb7077ff902db57b4590abbffcb1f57d/src/cmd/github_action.rs#L8-L11