nixguix: Load the content of sources.json as extrinsic snapshot metadata.
Test Plan
Tests will fail as they depend on swh.storage diffs: swh-storage!499 (closed) and swh-storage!500 (closed)
Migrated from D3667 (view on Phabricator)
Staging instance, all changes can be removed at any time
Tests will fail as they depend on swh.storage diffs: swh-storage!499 (closed) and swh-storage!500 (closed)
Migrated from D3667 (view on Phabricator)