nixguix: Filter out releases with URLs different from the expected one
This solves two problems:
- if the URL changes but the content doesn't, then the new snapshot would keep using the release with the old URL in its name.
- if there are two URLs pointing to the same content, the base loader would crash because it cannot know which one to pick.
Depends on !256 (closed). Resolves SWH-LOADER-CORE-115
Migrated from D6804 (view on Phabricator)