Always set valid in path-info --json output

Currently the valid key is only present when the path is invalid, which
makes checking path validity more complex than it should be. With this
change, the valid key can always be used to check if a path is valid

src/libstore/store-api.cc

@@ -855,6 +855,7 @@ json Store::pathInfoToJSON(const StorePathSet & storePaths,
             auto info = queryPathInfo(storePath);
 
             jsonPath["path"] = printStorePath(info->path);
+            jsonPath["valid"] = true;
             jsonPath["narHash"] = info->narHash.to_string(hashBase, true);
             jsonPath["narSize"] = info->narSize;