ci: correct copilot-collections permissions so that the job can create a PR if needed (#253)

The job ran successfully if there were no changes needed, but was
missing the permissions required to open a PR if changes were needed.

Author: tonyandrewmeyer

.github/workflows/copilot-collections-update.yml

@@ -6,7 +6,10 @@ on:
 
 permissions: {}
 
-jobs:  
-  check-update:  
+jobs:
+  check-update:
     # Pinned to main to get the latest logic, but the content comes from the .copilot-collections.yaml file.
     uses: canonical/copilot-collections/.github/workflows/auto_update_collections.yaml@main
+    permissions:
+      contents: write
+      pull-requests: write