CMakePM: Use staging dir only when selected

Fixes: QTCREATORBUG-29223
Change-Id: Ibc31b73ddda4f3906929ec1a4ff13eca84a704c8
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildstep.cpp

@@ -245,7 +245,7 @@ CMakeBuildStep::CMakeBuildStep(BuildStepList *bsl, Id id) :
             env.set("NINJA_STATUS", ninjaProgressString + "%o/sec] ");
         env.modify(m_userEnvironmentChanges);
 
-        if (m_useStaging)
+        if (m_useStaging && m_useStaging->value())
             env.set("DESTDIR", currentStagingDir());
     });