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>

Changed files
+6
pkgs
by-name
sv
svt-av1-psy
+6
pkgs/by-name/sv/svt-av1-psy/package.nix
···
cmake,
nasm,
libdovi,
+
unstableGitUpdater,
}:
stdenv.mkDerivation (finalAttrs: {
···
buildInputs = [
libdovi
];
+
+
passthru.updateScript = unstableGitUpdater {
+
branch = "master";
+
tagPrefix = "v";
+
};
meta = {
homepage = "https://github.com/psy-ex/svt-av1-psy";