Enable cron for actions.

.github/workflows/pythonpackage.yml

@@ -1,6 +1,10 @@
 name: Python package
 
-on: [push, pull_request]
+on:
+  push:
+  pull_request:
+  schedule:
+    - cron: '0 6 * * *'  # Daily 6AM UTC build
 
 jobs:
   build: