MAINT: schedule cache to run weekly (#201)

.github/workflows/cache.yml

@@ -1,11 +1,8 @@
 name: Build Cache [using jupyter-book]
 on:
-  push:
-    branches:
-      - main
   schedule:
-    # run cache monthly to prevent expiration
-    - cron:  '0 0 1 * *'
+    # Execute cache weekly at 3am on Monday
+    - cron:  '0 3 * * 1'
 jobs:
   cache:
     runs-on: quantecon-gpu