nix-daemon.nix: Add option nix.registry

This allows you to specify the system-wide flake registry. One use is
to pin 'nixpkgs' to the Nixpkgs version used to build the system:

  nix.registry.nixpkgs.flake = nixpkgs;

where 'nixpkgs' is a flake input. This ensures that commands like

  $ nix run nixpkgs#hello

pull in a minimum of additional store paths.

You can also use this to redirect flakes, e.g.

  nix.registry.nixpkgs.to = {
    type = "github";
    owner = "my-org";
    repo = "my-nixpkgs";
  };

flake.nix

@@ -25,8 +25,9 @@
           import ./nixos/lib/eval-config.nix (args // {
             modules = modules ++
               [ { system.nixos.versionSuffix =
-                    ".${lib.substring 0 8 self.lastModified}.${self.shortRev or "dirty"}";
+                    ".${lib.substring 0 8 (self.lastModifiedDate or self.lastModified)}.${self.shortRev or "dirty"}";
                   system.nixos.revision = lib.mkIf (self ? rev) self.rev;
+                  nix.registry.nixpkgs.flake = lib.mkDefault self;
                 }
               ];
           });