Merge pull request #11898 from adelez/download_json_file

Fix cloud_to_prod interop tests on kokoro

tools/internal_ci/helper_scripts/prepare_build_interop_rc

@@ -26,3 +26,8 @@ git submodule update --init
 # Set up gRPC-Go and gRPC-Java to test
 git clone --recursive https://github.com/grpc/grpc-go ./../grpc-go
 git clone --recursive https://github.com/grpc/grpc-java ./../grpc-java
+
+# Download json file.
+mkdir ~/service_account
+gsutil cp gs://grpc-testing-secrets/interop/service_account/GrpcTesting-726eb1347f15.json ~/service_account
+export GOOGLE_APPLICATION_CREDENTIALS=~/service_account/GrpcTesting-726eb1347f15.json