Migrate file-based AWS credentials caching to be compatible with aws-sdk-go-v2

The interface for configuring a custom credentials caching strategy is different in aws-sdk-go-v2. As part of migrating to aws-sdk-go-v2, `aws.Config` needs to be constructed with a file-based credentials cache, similar to the [existing one](https://github.com/weaveworks/eksctl/blob/main/pkg/credentials/filecache.go). 

Related to #4877