nixos/misc: Fix nixpkgs.config merge function

[infinisil]

Motivation for this change

Previously nested attrsets would override each other

With this module:

{ lib, ... }: {

  nixpkgs.config = lib.mkMerge [
    { foo.bar = true; }
    { foo.baz = true; }
  ];

}

Before it would evaluate to

{ foo = { baz = true; }; }

Now it evaluates to

{ foo = { bar = true; baz = true; }; }

Fixes a problem #66448 tries to workaround

[bkchr]

Can confirm that this works :)

[infinisil]

Backported to 19.03 in 68dd778 as well

[SomeoneSerge]

@infinisil Hiii, coming here from #456510... yours was the most recent to touch the merge function, is there some obvious reason we can't do extendModules?

[infinisil]

Just a guess, but it could be that extendModules didn't exist back then!