Execute Harbormaster buildable filtering properly from HarbormasterBuildSearchEngine

Summary:
Ref T11473. When running harbormaster.build.search with a buildables constraint, the constraint doesn't get passed to the Query and so currently has no effect.

This piece of logic was just accidentally omitted from D16356. It is probably not used anywhere today and doesn't show up in the UI, so it's easy to overlook (I missed it in review, too).

Test Plan: Ran harbormaster.build.search with a buildables constraint, got expected filtering.

Reviewers: yelirekim, chad

Reviewed By: chad

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T11473

Differential Revision: https://secure.phabricator.com/D16395