top-level: turn the screw

- Non-cross stdenvs are honest and assert that `crossSystem` is null

 - `crossSystem` is a mandatory argument to top-level/stage.nix, just like
   `system` and `platform`

 - Broken default arguments on stdenvs for testing are gone.

 - All stdenvs (but little-used stdenvNix) take the same arguments for easy
   testing.

pkgs/stdenv/custom/default.nix

@@ -2,6 +2,8 @@
 , system, platform, crossSystem, config
 }:
 
+assert crossSystem == null;
+
 rec {
   vanillaStdenv = import ../. {
     inherit lib allPackages system platform crossSystem;