Update cloudbuild with multiarch platforms

.cloudbuild.sh

+#! /bin/bash
+
+# Copyright 2020 The Kubernetes 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
+#
+#     http://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.
+
+
+: ${CSI_PROW_BUILD_PLATFORMS:="linux amd64; linux arm -arm; linux arm64 -arm64; linux ppc64le -ppc64le; linux s390x -s390x"}
+
+# shellcheck disable=SC1091
+. release-tools/cloudbuild.sh