workflows: Auto-refresh tasks container every week

We tend to forget about refreshing the tasks container, and thus fall
behind on browser and toolchain updates.

Run these every Saturday, so that they are ready for the Sunday night
runs of our projects' tasks-container-update workflows.

.github/workflows/build-tasks.yml

@@ -3,6 +3,9 @@ name: build-tasks
 on:
   # this is meant to be run on an approved PR branch for convenience
   workflow_dispatch:
+  # Run this on Saturday early morning (UTC), our projects try to update Sunday night
+  schedule:
+    - cron: '3 00 * * 6'
 
 # We derive a unique ID here to make sure that we don't get into a situation
 # where different runners pick different tags (eg: for builds near midnight, or