top-level: Close over fewer arguments for stdenv stages

This makes the flow of data easier to understand. There's little downside
because the args in question are already inspected by the stdenvs.

cross-compiling in particular is simpler because we don't need to worry
about overriding the config closed over by `allPackages`.

pkgs/top-level/default.nix

@@ -69,9 +69,10 @@ in let
   # deterministically inferred the same way.
   nixpkgsFun = newArgs: import ./. (args // newArgs);
 
-  # Partially apply some args for building bootstraping stage pkgs sets
+  # Partially apply some arguments for building bootstraping stage pkgs
+  # sets. Only apply arguments which no stdenv would want to override.
   allPackages = newArgs: import ./stage.nix ({
-    inherit lib nixpkgsFun config;
+    inherit lib nixpkgsFun;
   } // newArgs);
 
   stdenv = import ../stdenv {