build: Remove references to 'nix-setuid-helper'.

'nix-setuid-helper' vanished in commit d43eb499a (Jan. 2014.)

* pre-inst-env.in: Remove occurrences of 'NIX_SETUID_HELPER'.

pre-inst-env.in

@@ -1,7 +1,7 @@
 #!/bin/sh
 
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2012, 2013, 2014, 2015 Ludovic Courtès <ludo@gnu.org>
 #
 # This file is part of GNU Guix.
 #
@@ -45,11 +45,10 @@ export PATH
 
 NIX_ROOT_FINDER="$abs_top_builddir/nix/scripts/list-runtime-roots"
 NIX_SUBSTITUTERS="$abs_top_builddir/nix/scripts/substitute-binary"
-NIX_SETUID_HELPER="$abs_top_builddir/nix-setuid-helper"
 NIX_BUILD_HOOK="$abs_top_builddir/nix/scripts/offload"
 NIX_LIBEXEC_DIR="@abs_top_builddir@/nix/scripts" # for 'guix-authenticate'
 
-export NIX_ROOT_FINDER NIX_SETUID_HELPER NIX_SUBSTITUTERS	\
+export NIX_ROOT_FINDER NIX_SUBSTITUTERS	\
     NIX_BUILD_HOOK NIX_LIBEXEC_DIR
 
 # The 'guix-register' program.