Forgejo versions before 15 didn't support workflow expansion, and
"runs-on" was necessary in callers of reusable workflows. janware.com
now runs Forgejo 15.x, and the requirement is gone, so remove the config
option.
Quoting from https://forgejo.org/docs/latest/user/actions/reference/
runs-on is typically a required field. However, if a job defines
jobs.<job_id>.uses in order to reference a reusable workflow, then it
is optional. See jobs.<job_id>.uses for more information on this
behaviour.
[...]
It is recommended that jobs.<job_id>.runs-on is omitted when using
uses, as this will allow Forgejo to perform workflow expansion.
Workflow expansion results in the target workflow’s jobs appearing in
the UI as separate jobs. This provides an easier to understand
experience for accessing the logs of each job, and permits the jobs to
run on separate runners with their own runs-on fields.
Signed-off-by: Jan Lindemann <jan@janware.com>