Get environment variables values only if `Config._allow_environment_variables` is True

The issue was that `self.allow_environment_variables` is the class method, and it's always gonna be a true value because the method exists. Thus, it would get the env var value even if it was not requested.

This may break some code because people they didn't realize they need a call to `Config.allow_environment_variables()` to make env vars work.

If this is not the expected behavior, please let me know.