The returned source may actually consist of multiple schema sources from different directories, depending on which directories were given in `XDG_DATA_DIRS` and `GSETTINGS_SCHEMA_DIR`. For this reason, all lookups performed against the default source should probably be done recursively.
Without this in NixOS it will not work as intended, as we
have multiple entries in XDG_DATA_DIRS.
The docs on SettingsSchemaSource get_default say
Without this in NixOS it will not work as intended, as we have multiple entries in XDG_DATA_DIRS.