let a rerun of the nix pin job succeed when the pin already landed

.github/workflows/release.yml

@@ -220,6 +220,11 @@ jobs:
           git config user.name "github-actions[bot]"
           git config user.email "github-actions[bot]@users.noreply.github.com"
           git add nix/release.json
+          # A rerun after the pin already landed has nothing to commit, and the release is done.
+          if git diff --cached --quiet; then
+            echo "The flake already points at v$VERSION, nothing to push."
+            exit 0
+          fi
           git commit -m "point the nix package at v$VERSION"
           for attempt in 1 2 3 4 5; do
             git fetch origin main