use AWS_CONFIG_FILE if defined

[gumieri]

The option --set-ini assume the ~/.aws/config aws the only possible location for the config file, but there's a environment variable to change it: https://docs.aws.amazon.com/cli/v1/userguide/cli-configure-envvars.html#envvars-list (AWS_CONFIG_FILE).

Some people (like me) may use this to place the AWS config based on the standard "XDG Base Directory" (~/.config/). By ex.: AWS_CONFIG_FILE=$HOME/.config/aws/config.