Update submodules in grpc-tools kokoro scripts

tools/release/kokoro-grpc-tools.bat

@@ -1,5 +1,11 @@
 cd /d %~dp0
 cd ../..
 
+git submodule update --init --recursive
+
 set ARTIFACTS_OUT=%cd%/artifacts
-powershell -File ./packages/grpc-tools/build_binaries.ps1 || exit /b 1
+powershell -File ./packages/grpc-tools/build_binaries.ps1 || goto :error
+goto :EOF
+
+:error
+exit /b 1

tools/release/kokoro-grpc-tools.sh

@@ -4,6 +4,8 @@ base_dir=$(pwd)
 
 OS=$(uname)
 
+git submodule update --init --recursive
+
 case $OS in
 Linux)
   docker build -t kokoro-native-image tools/release/native