Remove push trigger from cache.yml - use weekly schedule only

Long-running cache builds should only run:
- Weekly on schedule (Monday 3am)
- Manual dispatch when needed
- Not on every main push (dependencies change rarely)

Author: mmcky

.github/workflows/cache.yml

@@ -1,7 +1,5 @@
 name: Build Cache [using jupyter-book]
 on:
-  push:
-    branches: [main]
   schedule:
     # Execute cache weekly at 3am on Monday
     - cron:  '0 3 * * 1'