Jenkinsfile: upload ostree images specifically

The ostree images were built after the ostree commit, which already
contained a `repo` folder created and uploaded to release server.

Signed-off-by: Frédéric Dalleau <frederic.dalleau@collabora.com>
Reviewed-by: Héctor Orón Martínez <hector.oron@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D7667