Use nightly images from knative-nightly (knative#279)

Part of knative/serving#2555.

Bonus: block running E2E tests using knative-nightly GCR as well.

scripts/e2e-tests.sh

@@ -358,7 +358,7 @@ function initialize() {
 
   # Safety checks
 
-  if [[ "${DOCKER_REPO_OVERRIDE}" =~ ^gcr.io/knative-releases/?$ ]]; then
+  if [[ "${DOCKER_REPO_OVERRIDE}" =~ ^gcr.io/knative-(releases|nightly)/?$ ]]; then
     abort "\$DOCKER_REPO_OVERRIDE is set to ${DOCKER_REPO_OVERRIDE}, which is forbidden"
   fi
 

scripts/library.sh

@@ -22,12 +22,12 @@
 readonly SERVING_GKE_VERSION=latest
 readonly SERVING_GKE_IMAGE=cos
 
-# Public images and yaml files.
-readonly KNATIVE_ISTIO_CRD_YAML=https://storage.googleapis.com/knative-releases/serving/latest/istio-crds.yaml
-readonly KNATIVE_ISTIO_YAML=https://storage.googleapis.com/knative-releases/serving/latest/istio.yaml
-readonly KNATIVE_SERVING_RELEASE=https://storage.googleapis.com/knative-releases/serving/latest/release.yaml
-readonly KNATIVE_BUILD_RELEASE=https://storage.googleapis.com/knative-releases/build/latest/release.yaml
-readonly KNATIVE_EVENTING_RELEASE=https://storage.googleapis.com/knative-releases/eventing/latest/release.yaml
+# Public latest stable nightly images and yaml files.
+readonly KNATIVE_ISTIO_CRD_YAML=https://storage.googleapis.com/knative-nightly/serving/latest/istio-crds.yaml
+readonly KNATIVE_ISTIO_YAML=https://storage.googleapis.com/knative-nightly/serving/latest/istio.yaml
+readonly KNATIVE_SERVING_RELEASE=https://storage.googleapis.com/knative-nightly/serving/latest/release.yaml
+readonly KNATIVE_BUILD_RELEASE=https://storage.googleapis.com/knative-nightly/build/latest/release.yaml
+readonly KNATIVE_EVENTING_RELEASE=https://storage.googleapis.com/knative-nightly/eventing/latest/release.yaml
 
 # Conveniently set GOPATH if unset
 if [[ -z "${GOPATH:-}" ]]; then