gatekeeper: set version (#397665)

pkgs/by-name/ga/gatekeeper/package.nix

···
     installShellFiles
   ];
 
+  ldflags = [
+    "-X github.com/open-policy-agent/gatekeeper/v3/pkg/version.Version=${version}"
+  ];
+
   subPackages = [ "cmd/gator" ];
 
   postInstall = ''