svt-av1-psy: provide update script

Upstream recommends to use the master branch:
<https://github.com/psy-ex/svt-av1-psy/blob/master/README.md#other-changes>

pkgs/by-name/sv/svt-av1-psy/package.nix

@@ -5,6 +5,7 @@
   cmake,
   nasm,
   libdovi,
+  unstableGitUpdater,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
@@ -35,6 +36,11 @@ stdenv.mkDerivation (finalAttrs: {
     libdovi
   ];
 
+  passthru.updateScript = unstableGitUpdater {
+    branch = "master";
+    tagPrefix = "v";
+  };
+
   meta = {
     homepage = "https://github.com/psy-ex/svt-av1-psy";
     description = "Scalable Video Technology AV1 Encoder and Decoder";