Adding gitspiegel-trigger workflow (#2135)

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