Enhancement: S3 backend: accept AWS_SESSION_TOKEN

Before, it was not possible to use s3 backend with AWS temporary security credentials(with AWS_SESSION_TOKEN).
This change gives higher priority to credentials.EnvAWS credentials provider.

https://github.com/restic/restic/issues/1477
https://github.com/restic/restic/pull/1479
https://github.com/restic/restic/pull/1647
