mpvScripts.mpv-slicing: add `updateScript`

pkgs/applications/video/mpv/scripts/mpv-slicing.nix

···
   buildLua,
   fetchFromGitHub,
   ffmpeg,
+  unstableGitUpdater,
 }:
 
 buildLua {
···
     rev = "d09c11227704c8d5bdaa2c799ef64dce881c63a7";
     hash = "sha256-MKoM0f74/XoctiHQVOB3LzFWtJXpsREfQh5icaebCJo=";
   };
+  passthru.updateScript = unstableGitUpdater { };
 
   postPatch = ''
     substituteInPlace slicing.lua \