Use `platform` package with instance role instead of credentials

[otiai10]

As Is

Because platform package requires $HOME/.aws/credentials file here, $HOME/.aws/credentials must be located even if the environment is granted instance profile by which it can use awsub.

To Be

When the environment where user issue awsub command has the instance profile with IAM Full Access and EC2 Full Access, user should use awsub without $HOME/.aws/credentials file.

it may help

• https://github.com/otiai10/aws-sdk-go-sandbox/blob/master/create-security-group/main.go#L30-L32