Make URL prefix in `_get_image_name`-function more flexible

Currently, [`_get_image_name`](https://github.com/google-deepmind/xmanager/blob/7a9cfd0b1edf777cd4b10b9f81ab3a8a8bbdc9ec/xmanager/cloud/build_image.py#L171-L175) hardcodes the `gcr.io/` prefix. It would be nice to allow custom prefixes to enable better control over the location, for example via `pkg.dev` repositories in Artifact Registry (which has since replaced the deprecated [Container Registry](https://cloud.google.com/artifact-registry/docs/transition/gcr-repositories)).