Be careful with EXPO_PUBLIC_ variables, as indicated in the environment variables documentation:

Do not store sensitive information in EXPO_PUBLIC_ variables, such as private keys. These variables will be visible in plain-text in your compiled app.

For me, after several unsuccessful attempts to create an environment variable using EAS CLI (eas env:create or eas env:push) to store an encryption key, I've found that the easiest way was actually to do it manually in the Expo's project page, as mentioned by @Ali Raza Dar in the previous answer and in the Expo's documentation.

Of course, this is only useful if you are using EAS for building ;).