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