nixpkgs: only pass pkgs_i686 argument on Linux

Nixpkgs added an assertion on pkgsi686Linux [1] to avoid evaluating it
pkgsi686Linux on non-Linux systems.

[1] ad20a4a1c3

modules/misc/nixpkgs.nix

@@ -144,7 +144,7 @@ in
   config = {
     _module.args = {
       pkgs = _pkgs;
-      pkgs_i686 = _pkgs.pkgsi686Linux;
+      pkgs_i686 = if _pkgs.stdenv.isLinux then _pkgs.pkgsi686Linux else {};
     };
   };
 }