buildRustPackage: allow passthru overrides

Don't ignore the passthru that could be passed to the derivation

pkgs/build-support/rust/default.nix

···
     runHook postInstall
   '';
 
-  passthru = { inherit cargoDeps; };
+  passthru = { inherit cargoDeps; } // (args.passthru or {});
 })