bug#18994: Daemon does not preserve supplementary groups of build users

[Ludovic Courtès]

Currently, the build environment made by the daemon does not preserve
supplementary groups of the build users.

Thus, even though the standalone Guix system sets /dev/kvm 660, owned by
root:kvm, and adds the build users to the kvm group, build users are
unable to access it.

This can be see with:

  (gexp->derivation "foo"
    #~(begin (mkdir #$output)(pk (stat:gid (stat "/dev/kvm")) (getgroups))))

The workaround for now is to make /dev/kvm 666.

Ludo’.