workflows/command-not-found-db-update: don't run on all PRs.

Let's be more selective about when we run this.

Author: MikeMcQuaid

.github/workflows/command-not-found-db-update.yml

@@ -1,12 +1,9 @@
 name: Command-not-found scheduled database updates
 
 on:
-  push:
-    branches:
-      - main
+  pull_request:
     paths:
       - .github/workflows/command-not-found-db-update.yml
-  pull_request:
   schedule:
     - cron: "0 0 * * *"
   workflow_dispatch: