workflows: Move tasks-container-update to Sunday night

We previously ran tasks-container-update and cockpit-lib-update at the
same time, which sometimes collides. Move it to the night between Sunday
and Monday instead, so that we have the updates ready for us to inspect
when we start our week.

.github/workflows/tasks-container-update.yml

@@ -1,7 +1,7 @@
 name: tasks-container-update
 on:
   schedule:
-    - cron: '0 2 * * 4'
+    - cron: '0 2 * * 1'
   # can be run manually on https://github.com/cockpit-project/starter-kit/actions
   workflow_dispatch:
 jobs: