S3 implementation does not use Role ARN provided by environment or profile

In [backend.rs](https://github.com/apache/opendal/blob/688fde20d8b9072432c5b3f9be40dd1703b98ade/core/src/services/s3/backend.rs#L908) for S3, the check is only for `self.config` value of a role to assume, and ignores the value loaded into `cfg`. This doesn't allow values from an environment variable or aws config profile to be used to define the role to assume, even though it seems like it's intended to.

Start in core/src/services/s3/backend.rs at the linked code around line 908, and trace how the role value is loaded into cfg versus self.config. Verify the role-selection path for environment variables and AWS config profiles; done means an externally provided Role ARN is used when no explicit self.config value is set.

Written by the indexing model from the issue text.