release: Don't copy container images into /ftp/

These are installed into /ociimages/ and stage onto the download site
at /OCI-IMAGES/; they don't belong in the same directory as the
"distribution" .txz files.

Fixes: d03c82c28da8 ("release: add optional OCI images")
MFC after: 1 minute
Sponsored by: Amazon

(cherry picked from commit 976e1d36be688166e140cbaded01d433c61e6d47)