Add a missing artifactIndex key to HarbormasterBuildArtifact

Summary: See PHI176. We issue a query with only artifactIndex from BuildTarget, but don't have an applicable key.

Test Plan: This isn't on the normal Harbormaster execution path so I'm not 100% sure I have a local repro, but will confirm with customer.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D18732