Optimize DummyStore::isValidPathUncached

See the API docs for the rationale of why this is needed.

src/libstore/dummy-store.cc

@@ -168,6 +168,15 @@ struct DummyStoreImpl : DummyStore
         callback(nullptr);
     }
 
+    /**
+     * Do this to avoid `queryPathInfoUncached` computing `PathInfo`
+     * that we don't need just to return a `bool`.
+     */
+    bool isValidPathUncached(const StorePath & path) override
+    {
+        return path.isDerivation() ? derivations.contains(path) : Store::isValidPathUncached(path);
+    }
+
     /**
      * The dummy store is incapable of *not* trusting! :)
      */