workflows: Switch release to cockpit/tasks container

The unit-tests container was dropped in
cockpit-project/cockpit@f16f1fc14b88c

Cherry-picked from cockpit-project/starter-kit@6e1427493a

.github/workflows/release.yml

@@ -8,7 +8,7 @@ jobs:
   source:
     runs-on: ubuntu-latest
     container:
-      image: ghcr.io/cockpit-project/unit-tests
+      image: quay.io/cockpit/tasks:latest
       options: --user root
     permissions:
       # create GitHub release