bug#19797: hydra: drvs deleted by GC, leads to aborted unrestartable builds

[Mark H Weaver]

Here are three examples in evaluation 102697 (the latest) of packages
that aborted because the garbage collector deleted their .drv files.
These builds cannot be restarted.

  http://hydra.gnu.org/build/237336
  http://hydra.gnu.org/build/237342
  http://hydra.gnu.org/build/237338

In all three cases, the error output looks like this:

--8<---------------cut here---------------start------------->8---
these derivations will be built:
  /gnu/store/ayjvjvmpaiwlj36vvfygdfbl48ql3gcb-wicd-1.7.3.tar.xz.drv
  /gnu/store/v88dz8vic2qj1jkv1a38s7x583klv3rq-wicd-1.7.3.drv
waiting for the big garbage collector lock...
cannot build missing derivation 
`/gnu/store/v88dz8vic2qj1jkv1a38s7x583klv3rq-wicd-1.7.3.drv'
error: build of `/gnu/store/v88dz8vic2qj1jkv1a38s7x583klv3rq-wicd-1.7.3.drv' 
failed
--8<---------------cut here---------------end--------------->8---

     Mark