Adds a `ref` input to workflow_dispatch so the nightly can be triggered
from main and run the scenarios against a feature branch that does not
yet carry the workflow file (e.g. feat/drive). Empty input falls back
to `github.ref`, so cron and bare dispatch are unchanged.
The regression-issue title and body now show the tested ref instead of
`github.sha`, which always resolves to the workflow's ref (main) under
workflow_dispatch and would otherwise mislead.
Edouard Vanbelle
fd467b2d3b