nixos/nix-daemon: allow registry paths to be... paths

Currently paths are handled by `types.package`, whose semantics are a
bit of a mess. In particular, it converts path values to derivations
using `toDerivation`, which will lead to problems when flake `outPath`s
become paths in https://github.com/NixOS/nix/pull/6530.

This change makes the "incompatible changes" section in the above PR
obsolete: `nix.registry.nixpkgs.flake = nixpkgs;` works as expected (the
flake is copied to the store).

nixos/modules/services/misc/nix-daemon.nix

@@ -414,6 +414,7 @@ in
               str
               int
               bool
+              path
               package
             ]);
           in