.github/workflows/pages.yml: only commit on main

Don't automatically commit changes to the Hugo site content unless
we are on the main branch. This is unnecessary and makes it tricky
to work on feature branches or pull requests.

.github/workflows/pages.yml

@@ -31,6 +31,9 @@ jobs:
         working-directory: site
       - name: Commit Hugo content
         uses: stefanzweifel/git-auto-commit-action@v4
+        # Only run if we are on the main branch. We don't want to create extra
+        # commits on feature branches or pull requests.
+        if: ${{ github.ref_name == 'main' }}
         with:
           commit_message: Automated commit with site updates
           # Only commit if there are changes to the site.