gitlab_runner: fix error about unbound variable

When there was an error i.e. with the image verification the loopdev
variable was unbound in the cleanup function. We fix this by defining
the variable as empty.

Signed-off-by: Christian Heusel <christian@heusel.eu>

roles/gitlab_runner/files/libvirt-executor-update-base-image

@@ -4,6 +4,8 @@ readonly libvirt_default_pool_path="/var/lib/libvirt/images"
 readonly arch_boxes_signing_key=/usr/local/lib/libvirt-executor/arch-boxes.asc
 readonly arch_boxes_fingerprint=1B9A16984A4E8CB448712D2AE0B78BF4326C6F8F
 
+loopdev=""
+
 cleanup() {
   set +o errexit