pkgs/config.nix: Fix infinite recursion when freeform config is strict in pkgs

https://github.com/NixOS/nixpkgs/issues/175196

The loop involved:

 - pkgs is strict in the config.nix freeformType's produced attrNames,
   as these are included in the config.
 - the attrNames were strict in all the direct attrValues, because of
   scanning for `mkIf`s.

`lazyAttrsOf` removes proper `mkIf` support, which we don't need here.

pkgs/top-level/config.nix

@@ -128,7 +128,7 @@ let
 in {
 
   freeformType =
-    let t = lib.types.attrsOf lib.types.raw;
+    let t = lib.types.lazyAttrsOf lib.types.raw;
     in t // {
       merge = loc: defs:
         let r = t.merge loc defs;