Fixing gitspiegel trigger workflow (#2679)

The first attept to use a workflow to protect GitLab CI from untrusted contributors failed, because GitHub doesn't pass secrets to workflows for PRs that originate from forks. 
 
This uses a different approach: instead of triggerring gitspiegel API directly from the workflow, we're just spawning an empty workflow with a specific path, and gitspiegel listens for `workflow_run` event to start mirroring.  

The idea is the same: for the first-time contributors, running workflows would require manual aciton and that would block mirroring. But this time, we don't need any secrets to make it work.