workflows/build-package.yaml: Remove "runs-on"

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>

.github/workflows/build-package.yaml

@@ -3,5 +3,4 @@ name: Test repo on all supported platforms
 on: [pull_request, push]
 jobs:
   CI:
-    runs-on: public-ci
     uses: ci/action-build-package/.github/workflows/reusable.yaml@master