GitHub Actions: Add workflow_dispatch trigger

[jkirk]

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