make-tarball.nix: Fix running as root

Since Nix now runs builds in a user namespace with uid == 0, this
triggered the message

  warning: the group ‘nixbld’ specified in ‘build-users-group’ does not exist

which make-tarball.nix turns into a fatal error. So clear
build-users-group.

http://hydra.nixos.org/build/44817408