Don't pin 'nixpkgs' in the system registry by default

Nixpkgs takes up a lot of disk space so we shouldn't do this by
default.

(cherry picked from commit 469f14ef0fade3ae4c07e4977638fdf3afc29e08)

flake.nix

···
               [ { system.nixos.versionSuffix =
                     ".${lib.substring 0 8 (self.lastModifiedDate or self.lastModified)}.${self.shortRev or "dirty"}";
                   system.nixos.revision = lib.mkIf (self ? rev) self.rev;
-                  nix.registry.nixpkgs.flake = lib.mkDefault self;
                 }
               ];
           });