[release-v0.26.1] Fix grpc-ping image ref (knative#873)

* [release-v0.26.1] Fix grpc-ping image ref

* Fix Makefile to build grpc-ping from vendor

Makefile

@@ -16,7 +16,7 @@
 
 CGO_ENABLED=0
 GOOS=linux
-TEST_IMAGES=$(shell find -L ./test/test_images ./vendor/knative.dev/serving/test/test_images/multicontainer -mindepth 1 -maxdepth 2 -type f -name "*.go" -exec dirname {} \;)
+TEST_IMAGES=$(shell find -L ./test/test_images ./vendor/knative.dev/serving/test/test_images/grpc-ping ./vendor/knative.dev/serving/test/test_images/multicontainer -mindepth 1 -maxdepth 2 -type f -name "*.go" -exec dirname {} \;)
 TEST=
 DOCKER_REPO_OVERRIDE=
 

test/test_images/grpc-ping/release.yaml

@@ -0,0 +1,16 @@
+# Copyright 2021 The Knative Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+- image: ko://knative.dev/serving/test/test_images/grpc-ping
+