Add cron config to GitHub Actions to run monthly

This will make sure the tests are green even if we're not making regular change
sto the code in this repo.

.github/workflows/main.yml

@@ -19,6 +19,16 @@ on:
     branches: [ main ]
   pull_request:
     branches: [ main ]
+  schedule:
+      # https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#schedule
+      #
+      #      ┌───────────── minute (0 - 59)
+      #      │ ┌───────────── hour (0 - 23)
+      #      │ │ ┌───────────── day of the month (1 - 31)
+      #      │ │ │ ┌───────────── month (1 - 12 or JAN-DEC)
+      #      │ │ │ │ ┌───────────── day of the week (0 - 6 or SUN-SAT)
+      #      │ │ │ │ │
+    - cron: '1 2 3 * *'  # run at 2:01 every month on the 3rd day
 
 jobs:
   build: