This makes the forgejo action more similar to the emerging standard in
the lab. The runs_on value should be enough to have it picked up by the
runner, as long as the repo lives in a priviledged org.
Although we do not have annexed content, the checkout action is switched
to datalad-clone-action following mih's suggestion to eat more of our
dog food.
What's missing is a schedule specification, and possibly a qualifier
constraing it to running in specific orgs / repos (but the deployment
repo is TBD).
