pkgs/top-level: expose current overlays in pkgs

This enables inspection of the currently used overlays. Useful for
usecases where nixpkgs is imported multiple times.
eg. different channels

	self: super:
	let
	  latest = import <nixpkgs-trunk> {
	    inherit (super) config overlays;
	  };
	in
	{
	  hello-custom-latest = latest.hello-custom;
	}