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

tests/ci-install.sh

@@ -4,6 +4,7 @@ set -euo pipefail
 set -x
 
 NULL=
+: "${ci_distro:=debian}"
 : "${ci_docker:=}"
 : "${ci_in_docker:=no}"
 : "${ci_suite:=jessie}"