add flake update

flake.nix

@@ -27,6 +27,7 @@
         type = "app";
         program = "${pkgs.writeShellScript "nixos-diff.sh" ''
           ${pkgs.git}/bin/git pull --ff-only
+          ${pkgs.nix}/bin/nix flake update
           ${pkgs.nixos-rebuild}/bin/nixos-rebuild build --log-format internal-json -v |& ${pkgs.nix-output-monitor}/bin/nom --json
           ${pkgs.nvd}/bin/nvd diff /run/current-system ./result
         ''}";