serving: E2E tests fail on Prow with newer ko versions

When making Prow use go 1.11.1 and Cloud SDK 225, E2E tests started failing with different errrors:

https://gubernator.knative.dev/build/knative-prow/pr-logs/pull/knative_serving/2468/pull-knative-serving-integration-tests/1062426675620352001/

W1113 19:37:16.237] 2018/11/13 19:37:16 error processing import paths in "config/webhook.yaml": BLOB_UNKNOWN: "Manifest \"sha256:e392d2a37aeba7425e33c18b57a1bf4a4663e35f5d67445727cafe2527edca6f\" blob \"sha256:f39c3f3ec4ec5beb7fbe9400562f4b186b5e784501d5d79f537d1ad376057b61\" unknown to registry."
...
I1113 19:37:40.400] >> Patching Istio
W1113 19:37:40.511] Error from server (NotFound): horizontalpodautoscalers.autoscaling "knative-ingressgateway" not found
I1113 19:37:40.612] ERROR: Knative Serving installation failed

https://gubernator.knative.dev/build/knative-prow/pr-logs/pull/knative_serving/2437/pull-knative-serving-integration-tests/1062427808908709890/

W1113 19:41:33.420] 2018/11/13 19:41:33 Unexpected error running "go build": exit status 2
W1113 19:41:33.421] # github.com/knative/serving/pkg/apis/serving/v1alpha1
W1113 19:41:33.422] pkg/apis/serving/v1alpha1/revision_validation.go:66:11: undefined: apis.ErrOutOfBoundsValue
W1113 19:41:33.422] 2018/11/13 19:41:33 error processing import paths in "config/activator.yaml": exit status 2
...
I1113 19:41:57.178] >> Patching Istio
W1113 19:41:57.282] Error from server (NotFound): horizontalpodautoscalers.autoscaling "knative-ingressgateway" not found
I1113 19:41:57.383] ERROR: Knative Serving installation failed

https://gubernator.knative.dev/build/knative-prow/logs/ci-knative-serving-continuous/1062420259731935234

W1113 19:13:15.335] 2018/11/13 19:13:15 error processing import paths in "config/999-cache.yaml": BLOB_UNKNOWN: "Manifest \"sha256:496427954201b3d3bb709160cc9bb44d87f0946ab6526e52cdd4745321c89e0f\" blob \"sha256:2992b18231ba9f736eadad4889e6cdcb1cc5e80b1646fc4d61a5d39afe09d391\" unknown to registry."
...
I1113 19:13:39.642] >> Patching Istio
W1113 19:13:39.758] Error from server (NotFound): horizontalpodautoscalers.autoscaling "knative-ingressgateway" not found
I1113 19:13:39.859] ERROR: Knative Serving installation failed

To reproduce it, you must start a Prow test job using prow-tests:v20181113-07b942c-dirty.