Add workflow scope to github.rb

[mindfulmonk]

I've recently tried bumping a cask version and got a recommendation to create a token with this message:

Or create a personal access token:
https://github.com/settings/tokens/new?scopes=gist,public_repo&description=Homebrew
As it was missing the workflow scope the bump failed.

To https://github.com/mindfulmonk/homebrew-cask.git
 ! [remote rejected]       bump-rekordbox-6.5.0-20210127201749 -> bump-rekordbox-6.5.0-20210127201749 (refusing to allow a Personal Access Token to create or update workflow `.github/workflows/automerge.yml` without `workflow` scope)

Previous pull request #10562

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