Removes use of ValueOrDefault function that is not always available on the buildbots.

PCbuild/pyproject.props

@@ -118,7 +118,8 @@ foreach (System.Diagnostics.Process p in System.Diagnostics.Process.GetProcesses
       </Code>
     </Task>
   </UsingTask>
-  <Target Name="KillPython" BeforeTargets="InitializeBuildStatus" Condition="$([msbuild]::ValueOrDefault($(KillPython), 'false'))">
+  
+  <Target Name="KillPython" BeforeTargets="InitializeBuildStatus" Condition="'$(KillPython)' == 'true'">
     <Message Text="Killing any running python.exe instances..." Importance="high" />
     <KillPython FileName="$(OutDir)python$(PyDebugExt).exe" />
   </Target>

Tools/buildbot/clean.bat

@@ -16,7 +16,7 @@ if "%1" == "x64" (
 call "%pcbuild%\env.bat" %vcvars_target%
 
 echo.Attempting to kill Pythons...
-msbuild /v:m /nologo /target:KillPython "%pcbuild%\pythoncore.vcxproj" /p:Configuration=Release /p:Platform=%platform%
+msbuild /v:m /nologo /target:KillPython "%pcbuild%\pythoncore.vcxproj" /p:Configuration=Release /p:Platform=%platform% /p:KillPython=true
 
 echo Deleting .pyc/.pyo files ...
 del /s "%root%\Lib\*.pyc" "%root%\Lib\*.pyo"