echoip: add updateScript

pkgs/by-name/ec/echoip/package.nix

···
   fetchFromGitHub,
   makeWrapper,
   nixosTests,
+  nix-update-script,
 }:
 
 buildGoModule {
···
 
   passthru = {
     tests = { inherit (nixosTests) echoip; };
+    updateScript = nix-update-script { extraArgs = [ "--version=branch" ]; };
   };
 
   meta = {