Fix the path to unified OStree repository.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>
Reviewed-by: Emanuele Aina <emanuele.aina@collabora.co.uk>
Reviewed-by: Martyn Welch <martyn.welch@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D7554