That error is now fixed in FIX: Use GlobalSetting values instead of ENV variables in migrate_to_s3 · discourse/discourse@60790eb · GitHub
Unfortunately, the Minio setup is currently broken, there are plans in the next few weeks to pull the SiteSetting s3_force_path_style that was introduced to support the Minio style urls.
Minio now supports the standard url structure that we use for S3. After the Minio-specific site setting is removed, the migration tasks will be tested and hopefully will work 
The migrate_to_s3 task will not work properly for Minio urls till this work is done.