lister.nixguix: Propagate the origin reference to the loader

Without this, the loader will fail.

Refs. swh/meta#4979

swh/lister/nixguix/lister.py

@@ -626,15 +626,18 @@ class NixGuixLister(StatelessLister[PageResult]):
     def artifact_to_listed_origin(self, artifact: Artifact) -> Iterator[ListedOrigin]:
         """Given an artifact (tarball, file), yield one ListedOrigin."""
         assert self.lister_obj.id is not None
+        loader_arguments = {
+            "checksums": artifact.checksums,
+            "checksum_layout": artifact.checksum_layout.value,
+            "fallback_urls": artifact.fallback_urls,
+        }
+        if artifact.ref:
+            loader_arguments["ref"] = artifact.ref
         yield ListedOrigin(
             lister_id=self.lister_obj.id,
             url=artifact.origin,
             visit_type=artifact.visit_type,
-            extra_loader_arguments={
-                "checksums": artifact.checksums,
-                "checksum_layout": artifact.checksum_layout.value,
-                "fallback_urls": artifact.fallback_urls,
-            },
+            extra_loader_arguments=loader_arguments,
         )
 
     def get_origins_from_page(

swh/lister/nixguix/tests/test_lister.py

@@ -359,6 +359,9 @@ def test_lister_nixguix_ok(datadir, swh_scheduler, requests_mock):
         # no last update is listed on those manifests
         assert listed_origin.last_update is None
 
+        if listed_origin.visit_type in {"git-checkout", "svn-export", "hg-checkout"}:
+            assert listed_origin.extra_loader_arguments["ref"] is not None
+
         mapping_visit_types[listed_origin.visit_type] += 1
 
     assert dict(mapping_visit_types) == expected_visit_types