this halves sandbox launch overhead, reducing the build time for 3000
trivial runCommand derivations on our machine from 80 seconds to 44s.
as a nice side effect we also get better error message propagation in
some cases, most notably setgroups failing when run in lix sandboxes.
Change-Id: Ia7c50a844915d9b8a20475b90b2d0179fd2fff34
eldritch horrors
56988d8605