Add env name to the "aws-exports.js"

**Is your feature request related to a problem? Please describe.**
I need to access the enviornment name of my backend in the frontend to display different features.

**Describe the solution you'd like**
Add the value "env" in the aws-exports.js which is generated by the Amplify CLI
(And when possible also to the build spec as a native Env to use it in this deployment as well)

**Describe alternatives you've considered**
Query the backend stack of amplify or replace a config file during the build process

Thx!