How to set private environment variables on Firebase?

Question

With the disconnection of Divshot, I recently switched several applications to Firebase hosting. Some of these applications connect to external APIs, so I need a way to store private environment variables (for example, a secret key for S3 access) on the Firebase host - who has any ideas? There this article is https://www.firebase.com/blog/2015-10-29-managing-development-environments.html - but this is only for non-private environment variables.

Thanks!