pkgs/top-level/stage.nix: don't override overlays and config in nixpkgsFun

`nixpkgsFun` already sets them via `args`. Doing this also introduces unexpected
hard to debug errors, see the patch.

pkgs/top-level/default.nix

@@ -98,6 +98,14 @@ in let
   # compiling toolchains and 32-bit packages on x86_64). In both those cases we
   # want the provided non-native `localSystem` argument to affect the stdenv
   # chosen.
+  #
+  # NB!!! This thing gets its `config` argument from `args`, i.e. it's actually
+  # `config0`. It is important to keep it to `config0` format (as opposed to the
+  # result of `evalModules`, i.e. the `config` variable above) throughout all
+  # nixpkgs evaluations since the above function `config0 -> config` implemented
+  # via `evalModules` is not idempotent. In other words, if you add `config` to
+  # `newArgs`, expect strange very hard to debug errors! (Yes, I'm speaking from
+  # experience here.)
   nixpkgsFun = newArgs: import ./. (args // newArgs);
 
   # Partially apply some arguments for building bootstraping stage pkgs