Only cleanup at end of job

* allows using the repos cloned into the workspace beforehand for
  gerrit notifications, required for checkout via waf because there
  the repos reside in the workspace and would be deleted prematurely
  without this change
* requires contiguous allocation of an agent, but the verification
  is not dominating anymore, therefore not much parallelization
  drawback expected

Change-Id: I06f8b826303fbf34dd58b026f7270eed6ab1b870