Fix Ponder Answer query joins, builtins

Summary: Fixes T9234. The joins method was still the old method and the builtin was calling the wrong key.

Test Plan: Test authored builtin, custom search

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9234

Differential Revision: https://secure.phabricator.com/D13953