CMake: Fix Clear Cache when builddir is not persistent yet.

Change-Id: I4dc2c5889845b13ebe167b89e78b104f8805a1f6
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cmakeprojectmanager/builddirmanager.cpp

@@ -231,8 +231,8 @@ void BuildDirManager::parse()
 
 void BuildDirManager::clearCache()
 {
-    auto cmakeCache = Utils::FileName(buildDirectory()).appendPath(QLatin1String("CMakeCache.txt"));
-    auto cmakeFiles = Utils::FileName(buildDirectory()).appendPath(QLatin1String("CMakeFiles"));
+    auto cmakeCache = Utils::FileName(workDirectory()).appendPath(QLatin1String("CMakeCache.txt"));
+    auto cmakeFiles = Utils::FileName(workDirectory()).appendPath(QLatin1String("CMakeFiles"));
 
     const bool mustCleanUp = cmakeCache.exists() || cmakeFiles.exists();
     if (!mustCleanUp)