Loading from external providers are discouraged. Please see parent tasks and J65.
In your case possible solutions would be:
- Use https://simonwhitaker.github.io/github-fork-ribbon-css/, we provide the css on https://tools-static.wmflabs.org/cdnjs/ajax/libs/github-fork-ribbon-css/
- https://tools.wmflabs.org/static-browser/ may have provided this image, but the service may be down at the moment
- Host the image file in your tool