test images: Image Promoter sed fix

The image used by the Image Promoter (gcr.io/k8s-testimages/gcb-docker-gcloud:v20190906-745fed4)
is based on busybox, and thus, the sed binary is actually busybox. image-util.sh calls
kube::util::ensure-gnu-sed several times, which ensures that a GNU sed binary exists
(it checks by greping GNU in its --help output). Obviously, it won't match the busybox sed
binary. But the sed usage in image-util.sh is fairly simple, and the busybox sed is sufficient.

This was previously fixed in: kubernetes#87188, but it was reverted by kubernetes#87653 as it was failing
on Mac (sed does not exist). This commit fixes that issue as well.

Author: claudiubelu

hack/lib/util.sh

@@ -700,7 +700,8 @@ function kube::util::ensure_dockerized {
 function kube::util::ensure-gnu-sed {
   # NOTE: the echo below is a workaround to ensure sed is executed before the grep.
   # see: https://github.com/kubernetes/kubernetes/issues/87251
-  if LANG=C sed --help 2>&1 | grep -q "GNU\|BusyBox"; then
+  sed_help="$(LANG=C sed --help 2>&1 || true)"
+  if echo "${sed_help}" | grep -q "GNU\|BusyBox"; then
     SED="sed"
   elif command -v gsed &>/dev/null; then
     SED="gsed"