osbs: only handle archives of type "image"
Prior to this change, if a container image used Cachito with OSBS, then
OSBS would store additional "remote-sources" files in the Koji archives
for the build. Pungi cannot parse the metadata for these archive
entries, so it would crash in add_metadata():
File "pungi/phases/osbs.py", line 81, in process
self.worker(compose, variant, config)
File "pungi/phases/osbs.py", line 141, in worker
nvr, archive_ids = add_metadata(variant, task_id, compose, scratch)
File "pungi/phases/osbs.py", line 447, in add_metadata
arch = archive["extra"]["image"]["arch"]
KeyError: 'image'
Tell Koji to only return container image archives, and ignore these
remote-source archives.
Signed-off-by: Ken Dreyer <kdreyer@redhat.com>