top-level: Do not splice pkgs, buildPackages or *Platform

- `pkgs` is self-similar, and thus already spliced
 - `buildPackages` is an ingredient of splicing and should be kept as is
 - The platforms are not packages or package sets and couldn't be spliced

There's probably other things that shouldn't be spliced too. The best long-
term solution is simply to stop splicing altogether.

pkgs/top-level/splice.nix

@@ -64,7 +64,11 @@ let
 
   splicedPackages =
     if actuallySplice
-    then splicer defaultBuildScope defaultRunScope
+    then splicer defaultBuildScope defaultRunScope // {
+      # These should never be spliced under any circumstances
+      inherit (pkgs) pkgs buildPackages
+        buildPlatform targetPlatform hostPlatform;
+    }
     else pkgs // pkgs.xorg;
 
 in