Problem/Motivation
The command to apply recipes is only requiring the path to the recipe and it is not possible to apply a recipe pointing to a different site than default.
Well...technically, it is already possible to apply recipes to a different site, by using the DRUPAL_DEV_SITE_PATH environment variable. But that's an undocumented, internal variable, meant primarily for testing, so there's no guarantee it will be supported for real-world use cases.
I propose we lift the functionality into an actual, supported, documented, user-facing option, making the recipe command's multisite support official.
The recipe command should accept a --site option, which is the name of a directory in sites that should be booted. This would be analogous to the --uri option in Drush.
For example:
php core/scripts/drupal recipe recipes/my-recipe --site=example.com
If the option is not present, default is assumed.