util-linux: improve purity by using login from shadow

replacing shutdown in postPatch phase is not necessary as rtcwake was already
patched to use the search path (the only user of shutdown)

pkgs/os-specific/linux/util-linux/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchurl, pkgconfig, zlib, fetchpatch
+{ lib, stdenv, fetchurl, pkgconfig, zlib, fetchpatch, shadow
 , ncurses ? null, perl ? null, pam, systemd, minimal ? false }:
 
 stdenv.mkDerivation rec {
@@ -17,12 +17,9 @@ stdenv.mkDerivation rec {
 
   outputs = [ "bin" "dev" "out" "man" ];
 
-  #FIXME: make it also work on non-nixos?
   postPatch = ''
-    # Substituting store paths would create a circular dependency on systemd
     substituteInPlace include/pathnames.h \
-      --replace "/bin/login" "/run/current-system/sw/bin/login" \
-      --replace "/sbin/shutdown" "/run/current-system/sw/bin/shutdown"
+      --replace "/bin/login" "${shadow}/bin/login"
   '';
 
   crossAttrs = {