erlcloud_aws:auto_config/0 fails to parse the credentials file

`aws_security_token` was renamed to `aws_session_token`

This resolved issue was related to the environment variables: https://github.com/erlcloud/erlcloud/issues/439

It still doesn't work with the .aws/credentials file.
AWS_SESSION_TOKEN is read correctly from the env variables, but not from the credentials file.

https://docs.aws.amazon.com/cli/latest/userguide/cli-configure-files.html