broken shields.io image on Github Enterprise

The backport pull request should be linked directly when running on Github Enterprise (i.e. when a custom `gitHostname` is configured) to avoid broken shields.io images:

<img width="605" height="223" alt="Image" src="https://github.com/user-attachments/assets/b236447d-3cfc-42c5-af09-4e2ba2f7b958" />