Pass TargetsWindowsArg to wrapper build in build-test.cmd

Helix builds tests on windows and runs them on unix using the xunit
wrappers. When cross-building the wrappers like this, TargetsWindows
is set to false by the test build pipeline. This variable ensures that
the wrapper uses correct directory separators when invoking the test
.sh file.

build-test.cmd

@@ -376,10 +376,13 @@ if defined __RuntimeId (
 )
 
 set TargetsWindowsArg=
+set TargetsWindowsMsbuildArg=
 if "%__TargetsWindows%"=="1" (
     set TargetsWindowsArg=-TargetsWindows=true
+    set TargetsWindowsMsbuildArg=/p:TargetsWindows=true
 ) else if "%__TargetsWindows%"=="0" (
     set TargetsWindowsArg=-TargetsWindows=false
+    set TargetsWindowsMsbuildArg=/p:TargetsWindows=false
 )
 
 echo %__MsgPrefix%Creating test overlay...
@@ -436,7 +439,7 @@ set __msbuildWrn=/flp1:WarningsOnly;LogFile="%__BuildWrn%"
 set __msbuildErr=/flp2:ErrorsOnly;LogFile="%__BuildErr%"
 
 REM Build wrappers using the local SDK's msbuild. As we move to arcade, the other builds should be moved away from run.exe as well.
-call %__DotnetHost% msbuild %__ProjectDir%\tests\runtest.proj /p:BuildWrappers=true !__msbuildLog! !__msbuildWrn! !__msbuildErr! %__msbuildArgs% %__unprocessedBuildArgs%
+call %__DotnetHost% msbuild %__ProjectDir%\tests\runtest.proj /p:BuildWrappers=true !__msbuildLog! !__msbuildWrn! !__msbuildErr! %__msbuildArgs% %TargetsWindowsMsbuildArg% %__unprocessedBuildArgs%
 if errorlevel 1 (
     echo Xunit Wrapper build failed
     exit /b 1