Adding gitspiegel-trigger workflow

GitHub has a setting that requires manual click for executing GHA on the branch, for the first-time contributors: https://docs.github.com/en/actions/managing-workflow-runs/approving-workflow-runs-from-public-forks.

After this PR, gitspiegel will respect that setting.
So, for PRs from first-time contributors, gitspiegel won't do mirroring until the button in PR is clicked.   
More info: https://github.com/paritytech/gitspiegel/issues/169