How to have an environment specific drush.yml? I'm using Drush 9. With the old drushrc.php I could easily include a local.drushrc.php that dynamically set the site url. How can I do it now?

This is similar to How can i set site url dynamically in Drush-9?. However I don't want to pass the url in every Drush command. I think of something like below in ../drush/drush.yml:

drush:
  paths:
    config:
      - ${env.home}/local.drush.yml


But I'd need another variable that point to the Drupal site directory (sites/default in a monosite, sites/CURRENT-SITE in a multisite). Does such a variable exist?
Reading https://github.com/drush-ops/drush/blob/master/examples/example.drush.yml it seems you can put a site specific drush.yml in a sites directory. It is merged with the global file(s).

PS! I haven't tested this myself.