ci-install: add ci_distro
ci-install: add ci_distro

Otherwise, we'll fail (due to set -u) if this parameter variable isn't
passed.

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>

Closes: #600
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree