deps/eigen3: Export Eigen3_DIR instead of CMAKE_PREFIX_PATH

I had some issues with Eigen3 in Simox. In particular, the system Eigen3 package was still being used and consequently, the old version did not provide the Eigen3::Eigen CMake target. This is mainly/also due to the FindEigen3.cmake file present in Simox (this should definitely be removed). Therefore, this change here does not solve the actual problem but anyways, the CMAKE_PREFIX_PATH should not be used.

Changelog entry: deps/eigen3: avoiding usage of CMAKE_PREFIX_PATH. using Eigen3_DIR instead.