Run CI at 4am every day

Ensures the build still works during periods of inactivity.

Note that:

> In a public repository, scheduled workflows are automatically disabled when no
> repository activity has occurred in 60 days.
-- https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#schedule

.github/workflows/ci.yml

@@ -4,6 +4,8 @@ on:
   push:
     branches: [ main ]
     tags: [ '*' ]
+  schedule:
+    - cron: '0 4 * * *'
 
 jobs: