Add a missing key to HarbormasterBuildArtifact

Summary: Fixes T10192. This key improves some common queries and is not currently present.

Test Plan: See discussion in T10192. Verified current query plan of real queries is garbage and improved by adding this key.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10192

Differential Revision: https://secure.phabricator.com/D15075