Since named entries can be overridden now based on ordering now merging
is only a problem for removing one of the default search paths, in which
case a higher priority eg. mkForce can be used.
{
nix.nixPath = [{ darwin-config = "/darwin.nix"; }];
}
Will result in
[ "darwin-config=/darwin.nix" "/nix/var/nix/profiles/per-user/root/channels" "$HOME/.nix-defexpr/channels" ]
Fixes#137
