nix-daemon service: Don't have the output in the `nix.package' option

1) It unnecessarily exposes implementation details.
2) It breaks all existing configs that have e.g.
`nix.package = pkgs.nixUnstable;`.

Changed files
+2 -2
nixos
modules
services
+2 -2
nixos/modules/services/misc/nix-daemon.nix
···
package = mkOption {
type = types.package;
-
default = pkgs.nix.out;
-
defaultText = "pkgs.nix.out";
+
default = pkgs.nix;
+
defaultText = "pkgs.nix";
description = ''
This option specifies the Nix package instance to use throughout the system.
'';