chore(ci): downgrade github actions runner version

downgrade the ubuntu version in effort to get the old cgroups mounts
back for CI. There seems to be a problem with the new version on
ubuntu-latest.

See: systemd/systemd#13477
See: docker/for-linux#219

.github/workflows/release.yml

@@ -26,7 +26,7 @@ jobs:
   release:
     name: release
     needs: test
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - name: Checkout
         uses: actions/checkout@v2