snis-assets: add update script

Signed-off-by: Marcin Serwin <marcin@serwin.dev>

pkgs/by-name/sn/snis-assets/package.nix

@@ -2,6 +2,7 @@
   lib,
   stdenv,
   fetchFromGitHub,
+  nix-update-script,
 }:
 stdenv.mkDerivation (finalAttrs: {
   pname = "snis_assets";
@@ -25,6 +26,8 @@ stdenv.mkDerivation (finalAttrs: {
     runHook postInstall
   '';
 
+  passthru.updateScript = nix-update-script { };
+
   meta = {
     description = "Assets for Space Nerds In Space, a multi-player spaceship bridge simulator";
     homepage = "https://smcameron.github.io/space-nerds-in-space/";