all: make Kokoro show actual Gradle results (#3208)

diff --git a/buildscripts/kokoro/windows.bat b/buildscripts/kokoro/windows.bat
index b7d3a89..3c92043 100644
--- a/buildscripts/kokoro/windows.bat
+++ b/buildscripts/kokoro/windows.bat
@@ -32,6 +32,7 @@
 echo vcProtobufInclude=%ESCWORKSPACE%\\grpc-java-helper\\protobuf-%PROTOBUF_VER%\\cmake\\build\\include>> gradle.properties
 
 cmd.exe /C "%WORKSPACE%\gradlew.bat build"
+set GRADLEEXIT=%ERRORLEVEL%
 
 @rem Rename test results .xml files to format parsable by Kokoro
 @echo off
@@ -41,4 +42,4 @@
 )
 @echo on
 
-exit %%ERRORLEVEL%%
+exit %GRADLEEXIT%