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