[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.
4 jobs for !94 with feature/find-master-grid-with-installed-stable-grid in 16 minutes and 23 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Test
passed #150523
duneci
debian:11--gcc-9-17

00:16:23

passed #150526
duneci
debian:11--gcc-9-17:headercheck

00:05:23

passed #150525
duneci
ubuntu:18.04--clang

00:06:32

passed #150524
duneci
ubuntu:18.04--gcc-7-17

00:07:27