iwyu.yml: run the workflow once a week [skip ci] (danmar#5726)

This will run the workflow every Monday at 00:00. This makes sure it
doesn't break and also provides some regular output.

.github/workflows/iwyu.yml

@@ -2,7 +2,10 @@
 # Environment reference https://help.github.com/en/actions/reference/virtual-environments-for-github-hosted-runners
 name: include-what-you-use
 
-on: workflow_dispatch
+on:
+  schedule:
+    - cron: '0 0 * * 0'
+  workflow_dispatch:
 
 permissions:
   contents: read