Use default GKE version for test cluster
`gke-latest` points to head, and these images might not yet be available for platforms other than Linux (e.g., Mac).
Fixes
# called from command line.
# Default GKE version to be used with Knative Serving
-readonly SERVING_GKE_VERSION=latest
+readonly SERVING_GKE_VERSION=default
readonly SERVING_GKE_IMAGE=cos
# Public images and yaml files.
