pkgs-lib: Make `lib` overlays be propagated

This is useful because the tests in `pkgs-lib` can mock out certain
`lib` functions like this using a `lib` overlay.

pkgs/top-level/all-packages.nix

···
   writers = callPackage ../build-support/writers { };
 
   # lib functions depending on pkgs
-  inherit (import ../pkgs-lib { inherit lib pkgs; }) formats;
+  inherit (import ../pkgs-lib {
+    # The `lib` variable in this scope doesn't include any applied lib overlays,
+    # `pkgs.lib` does.
+    inherit (pkgs) lib;
+    inherit pkgs;
+  }) formats;
 
   testers = callPackage ../build-support/testers { };