fix(gc): fix cleaning deduped blobs because they have the modTime of
the original blobs, fixed by updating the modTime when hard linking
the blobs.
fix(gc): failing to parse rootDir at zot startup when using s3 storage
because there are no files under rootDir and we can not create empty dirs
on s3, fixed by creating an empty file under rootDir.
Signed-off-by: Petu Eusebiu <peusebiu@cisco.com>