GitHub Actions: Add workflow_dispatch trigger

GitHub Actions: Add workflow_dispatch trigger

To manually run a workflow,  the workflow must be configured to run on
the workflow_dispatch event.

We want that all our GitHub Actions can be triggered manually.

See:

* https://docs.github.com/en/actions/using-workflows/manually-running-a-workflowk
* https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch