nixguix: Override known_artifacts to filter out evaluation branch
This branch does not have the right metadata format.
This actually is the reason the current nixguix loader refuses to load from a recent snapshot.
This has been patched in production and this is now loading.
Related to #2411 (closed)
Test Plan
- tox
- new run triggered on patch production
Migrated from D3178 (view on Phabricator)