CMake: fix not disconnected build action

The old code was disconnecting all signals from CMakeProjectPlugin
to m_buildTargetContextAction. The result was more and more triggered
actions.

Change-Id: I8d7db5852c03d5388870a7460efb3a8a28fd0749
Reviewed-by: Florian Apolloner <florian@apolloner.eu>
Reviewed-by: Alexander Drozdov <adrozdoff@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cmakeprojectmanager/cmakeprojectplugin.h

@@ -66,6 +66,7 @@ private:
     void updateContextActions(ProjectExplorer::Node *node, ProjectExplorer::Project *project);
 
     Utils::ParameterAction *m_buildTargetContextAction = nullptr;
+    QMetaObject::Connection m_actionConnect;
 };
 
 } // namespace Internal