`weed mini` sets the `-s3.iam.config` flag instead of `-s3.config`,
which populates `S3ApiServerOption.IamConfig`.
Previously, `NewIdentityAccessManagementWithStore` only checked
`option.Config`. This caused `weed mini` generated credentials (written
to a temp file passed via IamConfig) to be ignored, breaking S3 access
in mini mode even when environment variables were provided.
This change ensures we try to load the configuration from `IamConfig`
if `Config` is empty, restoring functionality for `weed mini`.
451b897d56