evcxr: add update script

pkgs/by-name/ev/evcxr/package.nix

@@ -11,6 +11,7 @@
   rustc,
   cmake,
   libiconv,
+  nix-update-script,
 }:
 
 rustPlatform.buildRustPackage rec {
@@ -82,6 +83,8 @@ rustPlatform.buildRustPackage rec {
       rm $out/bin/testing_runtime
     '';
 
+  passthru.updateScript = nix-update-script { };
+
   meta = with lib; {
     description = "Evaluation context for Rust";
     homepage = "https://github.com/google/evcxr";