reewide: Purge all uses stdenv.system and top-level system

It is deprecated and will be removed after 18.09.

pkgs/tools/misc/s6-portable-utils/default.nix

@@ -29,7 +29,7 @@ stdenv.mkDerivation rec {
   # Explicitly setting target ensures code can be compiled against a skalibs
   # binary built on a different version of darwin.
   # http://www.skarnet.org/cgi-bin/archive.cgi?1:mss:623:heiodchokfjdkonfhdph
-  ++ (stdenv.lib.optional stdenv.isDarwin "--build=${stdenv.system}");
+  ++ (stdenv.lib.optional stdenv.isDarwin "--build=${stdenv.hostPlatform.system}");
 
   postInstall = ''
     mkdir -p $doc/share/doc/s6-portable-utils/