ci(github): ignore status code 429

```
External link https://itch.io/docs/butler/login.html failed: Too Many Requests (status code 429)
```

.github/workflows/build.yml

@@ -23,5 +23,6 @@ jobs:
       - name: Run tests
         run: >
           bundle exec htmlproofer _site
+          --ignore-status-codes '429,999'
           --ignore-urls "/globalgamejam.org/,/newgrounds.com/,/unrealengine.com/"
           --no-check-external-hash