kopia: replace gitUpdater with nix-update script

gitUpdater does not update vendorHash and thus seems to prevent
auto-updates via r-ryantm.

pkgs/by-name/ko/kopia/package.nix

···
   lib,
   buildGoModule,
   fetchFromGitHub,
-  gitUpdater,
+  nix-update-script,
   installShellFiles,
   stdenv,
   testers,
···
   '';
 
   passthru = {
-    updateScript = gitUpdater { rev-prefix = "v"; };
+    updateScript = nix-update-script { };
     tests = {
       kopia-version = testers.testVersion {
         package = kopia;