Add more useful PHIDs to Harbormaster build variables

Summary: Ref T13609. Add the Object PHID (object being built), Container PHID (container of the object being built), Build PHID, and Buildable PHID to Harbormaster build variables.

Test Plan:
{F8448191}

{F8448192}

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13609

Differential Revision: https://secure.phabricator.com/D21569