How to rename .env variables in package.json?

What I have

I have multiple projects using Percy for Cypress where I set the PERCY_TOKEN env variable inside the .env file. The token is different for each project. In the CI I set different env variables for each project, but locally I have to do it in the .env file. Because of this, I have to edit the .env file whenever I change between projects.

Goal

I would like to set them in the .env file this way:

PROJECT_A_PERCY_TOKEN=tokenhash1PROJECT_B_PERCY_TOKEN=tokenhash2

So later I could rename these variables to PERCY_TOKEN, eliminating the need to constantly change the .env file.

What I tried

I'm trying to do this inside the package.json file's scripts property. Unfortunately echo $PROJECT_A_PERCY_TOKEN prints nothing. I know that I could create a shell/python/js script that parses the .env file, then passes the value back or calls npm run directly but I would like to do this without an external script.

Problem

It appears to me that I can't access the env variables inside package.json. Is there a way to rename the variable only using the npm script?