Test on GitHub Actions with the real Amazon S3 if needed

For the time being, `embulk-output-s3` is not tested with the real Amazon S3.

The Embulk project may want to have an AWS account maintained by the community.

Or, we may want to test with a "S3-compatible" local storage (ex. MinIO), but note that they may not be 100% S3-compatible. (For example in MinIO: https://min.io/docs/minio/linux/operations/concepts/thresholds.html#conflicting-objects)

The issue names no files, tests, or entry points. First clarify whether the project wants community-managed AWS credentials for real S3 or a MinIO-based alternative, then inspect the existing GitHub Actions configuration and test setup. Done means the chosen storage path runs reliably in CI and exercises embulk-output-s3 against it.

Written by the indexing model from the issue text.