When the env_auth option is enabled, the AWS SDK's session constructor
now loads configuration from ~/.aws/config and environment variables,
and credentials per the selected (or default) AWS_PROFILE's settings.
This is accomplished by **NOT** including any Credential provider in the
aws.Config passed to the session constructor: If the Config.Credentials
is non-nil, that will always be used and the user's configuration re
role_arn, credential_source, source_profile, etc... from the shared
config will be completely ignored.
(The conditional creation and configuration of the stscreds Credential
provider is complicated enough that it is not worth re-creating that
logic.)
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