fix sync script to account for subfolder bucket paths

[noyoshi]

• The sync script would fail if HF_CACHE_BUCKET is not just a bucket, but a bucket + a subfolder
• Added in extra check to ensure that if that is the case, it will still work.

The files variable will have everything except for the bucket, even the bucket's subfolder, so we should ensure that when we construct the S3 path we exclude any subfolders in the bucket env variable.