Rename HarbormasterBuildMessage "buildTargetPHID" to "receiverPHID"

Summary: Ref T13054. Companion storage change for D19062.

Test Plan: Applied migration and adjustments. Viewed messages in Harbormaster; created them with harbormaster.sendmessage; processed them with bin/phd debug task.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13054

Differential Revision: https://secure.phabricator.com/D19063