Prevent artifact key collision when builds are restarted

Summary: Ref T1049.  Because we no longer destroy artifacts when builds are restarted, we need the build generation number to be part of the artifact key, otherwise we get collisions when restarting builds that contain build steps that emit artifacts.

Test Plan: Ran it with a build plan of "Lease Host" and "Run Command", no longer got an artifact key crash.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T1049

Differential Revision: https://secure.phabricator.com/D10336