Update ci.yml: Handle pull for main branch

.github/workflows/ci.yml

@@ -48,7 +48,11 @@ jobs:
 
     - name: Pull latest changes
       run: |
-        git pull --rebase origin ${{ github.event.pull_request.head.ref }}
+        if [ "${{ github.event_name }}" = "pull_request" ]; then
+          git pull --rebase origin ${{ github.event.pull_request.head.ref }}
+        else
+          git pull --rebase origin main
+        fi
 
     - name: Push changes
       if: success()