does this work?

.github/workflows/stable.yml

@@ -9,6 +9,7 @@ jobs:
         toolchain: [nightly, stable]
         profile: [minimal, default]
     runs-on: ubuntu-latest
+    container: ubuntu
     # Only run on "pull_request" event for external PRs. This is to avoid
     # duplicate builds for PRs created from internal branches.
     if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository