Add weekly cron to CI (#257)

Author: kevmoo

.github/workflows/dart.yml

@@ -6,6 +6,8 @@ on:
       - main
       - master
   pull_request:
+  schedule:
+    - cron: "0 0 * * 0"
 defaults:
   run:
     shell: bash

mono_repo.yaml

@@ -1,5 +1,9 @@
+# See https://github.com/google/mono_repo.dart for details on this file
 self_validate: analyze_and_format
 
+github:
+  cron: "0 0 * * 0"
+
 merge_stages:
   - analyze_and_format
   - unit_test