How to provide preview channels on Firebase hosting with environment variables?

Question

I want to use preview channels on Firebase in order to share and test feature before they go live. Creating the preview channels has been simple but currently I am getting an error when using the preview channel url:

I currently use a .env file that store React environment variables that allow me to connect to firebase but when building and deploying for the preview channels these env variables can't seem to be accessed.

Any ideas on how I can get this to work please