[tests] `gradlew` needs JAVA_HOME set on Windows (#870)

Context: commit 3dc37379

Failing build at: https://devdiv.visualstudio.com/DevDiv/_build/index?buildId=999607

    ##[error]EXEC(0,0): Error : JAVA_HOME is not set and no 'java' command could be found in your PATH.

Export `%JAVA_HOME%` so that `gradlew` executes properly.

tests/CodeGen-Binding/Xamarin.Android.LibraryProjectZip-LibBinding/Xamarin.Android.LibraryProjectZip-LibBinding.targets

@@ -43,7 +43,7 @@
       Inputs="java\JavaLib\project.properties"
       Outputs="$(OutputPath)JavaLib.zip">
     <Exec
-        EnvironmentVariables="ANDROID_HOME=$(AndroidSdkDirectory)"
+        EnvironmentVariables="ANDROID_HOME=$(AndroidSdkDirectory);JAVA_HOME=$(JavaSdkDirectory)"
         Command=".\gradlew assembleDebug --stacktrace"
         WorkingDirectory="$(MSBuildThisFileDirectory)java\JavaLib"
     />