By omitting this option, we fall back to the hierarchy used by S3Client by
default. When defined, the provider chain will use the values of
AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY (and AWS_SESSION_TOKEN); in their
absence, the provider chain can retrieve credentials from a range of other
sources, including e.g. ECS credentials.
Although there are no longer any application reads from `env.AWS_ACCESS_KEY_ID`
and `env.AWS_SECRET_ACCESS_KEY`, they continue to serve a useful documentary
role.
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