Observe ASDF_CONCURRENCY

First of all, thank you for all the hard work so far.

I would like to inquire about support for [`ASDF_CONCURRENCY`](https://asdf-vm.com/manage/configuration.html#concurrency). Unless I'm missing something, it looks it was added in https://github.com/asdf-vm/asdf-ruby/commit/77df296, but then got lost during the transition to `ruby-build`.

Do you think we could reintroduce it by passing the value to `ruby-build` as [`MAKE_OPTS`](https://github.com/rbenv/ruby-build/blob/v20231014/bin/ruby-build#L589)? I'm hoping it is as simple as exporting before the build command:

```diff
+export MAKE_OPTS="-j $ASDF_CONCURRENCY"
echo "$patches" | "$(ruby_build_path)" ${opts} "$version" "$install_path"
```