[CMake] Find master dune-grid in home dir even with installed stable.

When not using dunecontrol but pure cmake, an installed dune-grid was preferred. In my case I was testing master and had 2.6 installed. The latter was found when running cmake directly.

This change fixes this by introducing the same trickery as for dune-common. Please note that this is not needed for modules that adhere to the usual workflow (without an explicit search for dependencies and relying on DuneMacros). But here a different approach was used and that probably has its reasons.