Delete artifacts when restarting build

Summary: Fixes T4336.  This updates the build engine to delete all artifacts when targets are being deleted.  This prevents conflicts when builds are restarted.

Test Plan: Restarted a build that had a lease host step and it didn't crash.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4336

Differential Revision: https://secure.phabricator.com/D8092