_stream: speed up bst artifact show

We only need to check the remotes for cached artifacts if we don't have them
cached locally.

Author: abderrahim

src/buildstream/_stream.py

@@ -797,8 +797,10 @@ def artifact_show(self, targets, *, selection=_PipelineSelection.NONE):
 
         self.query_cache(target_objects)
 
+        not_cached_locally = [element for element in target_objects if not element._cached()]
+
         if self._artifacts.has_fetch_remotes():
-            self._resolve_cached_remotely(target_objects)
+            self._resolve_cached_remotely(not_cached_locally)
 
         return target_objects