update build settings for addressing environment variables

[rishi-raj-jain]

fix #16

Officially recommended by https://docs.aws.amazon.com/amplify/latest/userguide/environment-variables.html#access-env-vars

[alexnguyennz]

Great thank you! I wonder if something changed, I definitely tried this (and all sorts of other variations) a few months ago.

[rishi-raj-jain]

Pleasure! Thank you for creating this amazing package. I am right now running this configuration to have deployed a sample e-commerce store: https://twitter.com/rishi_raj_jain_/status/1773085778710573063

[rishi-raj-jain]

Not sure about if something changed... I just went through the official docs and it seemed to work :)