Allow forcing Windows CI

Author: abgruszecki

.github/workflows/ci.yaml

@@ -119,6 +119,10 @@ jobs:
          || (
            github.event_name == 'push'
            && !startsWith(github.event.ref, 'refs/tags/sbt-dotty-')
+         )
+         || (
+           github.event_name == 'pull_request'
+           && contains(github.event.pull_request.body, '[test_windows_full]')
          )"
 
     steps: