Test on GitHub Actions with the real Amazon S3 if needed

For the time being, `embulk-input-s3` is not tested with the real Amazon S3. (The environment variables `EMBULK_S3_TEST_BUCKET`, `EMBULK_S3_TEST_ACCESS_KEY_ID`, and `EMBULK_S3_TEST_SECRET_ACCESS_KEY` need to be set.)

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)