workflow: really fix git diff call by using --cached

.gitea/workflows/update-flake.yaml

@@ -49,12 +49,9 @@ jobs:
       - name: Commit and push changes
         shell: bash
         run: |
-          git status
-
-          git diff
-
           git add flake.lock
-          git diff --quiet && echo "No changes to commit." && exit 0
+          git status
+          git diff --cached --quiet && echo "No changes to commit." && exit 0
 
           git commit -m "Update flake.lock $(date -I)"
           git push origin flake-updates