Make EnvironmentVariable a static file and remove variable_injector dependency

[remstos]

Possible alternative solutions to VI:
https://betterprogramming.pub/manage-secrets-in-your-ios-app-using-xcode-configuration-files-fbceb6e97f47
https://cocoacasts.com/swift-and-cocoa-fundamentals-protecting-the-secrets-of-your-mobile-application
https://nshipster.com/secrets/
https://medium.com/@robinmalhotra/secret-variables-in-xcode-and-your-ci-for-fun-and-profit-d387a50475d7

Using a simple Plist sounds easy enough...