How to use System.get_env inside my config.exs file?

This way it works for me but the problem is that I expose the keys explicitly and the keys must be in my .env file

aws_access_key_iam_user = "access_key"
aws_secret_access_key_iam_user = "secret_key"

config :ex_aws,
  json_codec: Jason,
  access_key_id: aws_acces_key_iam_user,
  secret_access_key: aws_secret_acces_key_iam_user

When I save the keys in my .env file

config :ex_aws,
    json_codec: Jason,
    access_key_id: System.get_env("AWS_ACCESS_KEY_IAM_USER"),
    secret_access_key: System.get_env("AWS_SECRET_ACCESS_KEY_IAM_USER")

The SignatureDoesNotMatch error indicates that the signature generated to authenticate the request to AWS does not match what the service expects.