Summary: Ref T9352. See D13635. Build targets can have variables already, but let builds have them too. This mostly enables future use cases (sub-builds, more sophisticated build triggers).
Test Plan: With a custom Herald rule + action like the one in T9352, updated a revision and saw it generate multiple builds with varying parameters.
Reviewers: chad, hach-que
Reviewed By: hach-que
Maniphest Tasks: T9352
Differential Revision: https://secure.phabricator.com/D14222
