[cache] Enable main and schedule for now (#29)

* cache should run weekly only

* enable main for now then disable

Author: mmcky

.github/workflows/cache.yml

@@ -1,5 +1,8 @@
 name: Build Cache [using jupyter-book]
 on:
+  push:
+    branches:
+      - main
   schedule:
     # Execute cache weekly at 3am on Monday
     - cron:  '0 3 * * 1'