Fix bump-kubevirtci.sh (#321)

Since line format was changed to allow overriding default value,
need to adapt the bump script.

Signed-off-by: Or Shoval <[email protected]>

Author: oshoval

hack/bump-kubevirtci.sh

@@ -3,6 +3,6 @@
 KUBEVIRTCI_TAG=$(curl -L -Ss https://storage.googleapis.com/kubevirt-prow/release/kubevirt/kubevirtci/latest)
 [[ ${#KUBEVIRTCI_TAG} != "18" ]] && echo "error getting KUBEVIRTCI_TAG" && exit 1
 
-sed -i "s/export KUBEVIRTCI_TAG=.*/export KUBEVIRTCI_TAG=${KUBEVIRTCI_TAG}/g" cluster/cluster.sh
+sed -i "s/\(KUBEVIRTCI_TAG:-\)[^}]*/\1${KUBEVIRTCI_TAG}/" cluster/cluster.sh
 
 git --no-pager diff cluster/cluster.sh | grep KUBEVIRTCI_TAG || true