Enhancement: Accept AWS_SESSION_TOKEN for the s3 backend

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
