CI: Run workflow each night

Author: amotl

.github/workflows/tests.yml

@@ -8,6 +8,10 @@ on:
   # Allow job to be triggered manually.
   workflow_dispatch:
 
+  # Run job each night.
+  schedule:
+    - cron: '0 4 * * *'
+
 # Cancel in-progress jobs when pushing to the same branch.
 concurrency:
   cancel-in-progress: true