Split up the onlabel_ workflow

[j-i-l]

See also t4d-gmbh/stubbed_versioning#106

Basically, we want to generate a single workflow that runs on > pull request > labeled, detects the label that was set and triggers the related workflow.

The related workflow then should run and trigger, also after a failure, a "post run" workflow that sets the appropriate ::passed or ::failed label.