We're at the point where there are enough Jenkins jobs for
Asterisk branches than even cleaned checkouts of Asterisk
will add up to more disk space than is available on the
in-memory workspace mount. Since we archive all relevent
artifacts anyway, there's no need to keep the workspace
around after the job finishes, whether it succeeds or fails.
Change-Id: I1cd3b73ebb045a987df0f62526d152a510210c39