libhwy: apply the new patch conditionally

Just temporarily to avoid the many rebuilds on staging-next branch.
It's a platform-specific bug.

pkgs/development/libraries/libhwy/default.nix

···
     rev = version;
     hash = "sha256-Gym2iHq5ws9kuG4HWSQndD8hVugV4USZt6dUFnEkLwY=";
   };
-  patches = [
+  patches = lib.optionals (with stdenv; isAarch64 && isLinux) [ # conditional, temporarily
     # backport for compilation issue on aarch64
     # https://github.com/google/highway/issues/1613
     (fetchpatch {