Avoid reading ~/.aws/config when using S3 provider

When using S3 storage, we sometimes have open_basedir errors due to aws-sdk-php trying to read ~/.aws/config which is out of open_basedir restrictions.
Christoph Wurst already added csm=false config in #21406 but it wasn't enough, we also need to set use_arn_region=false, added in this commit.

Signed-off-by: Florent <florent@coppint.com>