nixguix: Ensure to not use a redirection URL as an origin URL

Redirection URLs can be long and quite obscure in some cases (GitHub CDN
for instance) so ensure to use the redirected URL as origin URL.

Related to swh/meta#5090.

swh/lister/nixguix/lister.py

@@ -262,9 +262,7 @@ def is_tarball(
             if location:  # It's not always present
                 logger.debug("Location: %s", location)
                 try:
-                    # FIXME: location is also returned as it's considered the true
-                    # origin, true enough?
-                    return _is_tarball(location), location
+                    return _is_tarball(location), url
                 except ArtifactWithoutExtension:
                     logger.warning(
                         "Still cannot detect extension through location <%s>...",

swh/lister/nixguix/tests/test_lister.py

@@ -189,7 +189,7 @@ def test_is_tarball_complex_with_location_result(
     is_tar, origin = is_tarball(urls, requests)
     assert is_tar == expected_result
     if is_tar:
-        assert origin == fallback_url
+        assert origin == url
 
 
 @pytest.mark.parametrize(