Set the DUNE_PYTHON_WHEELHOUSE pyth even if CMAKE_INSTALL_PREFIX is set after the first run

summary

The variable DUNE_PYTHON_WHEELHOUSE contains the path where python files from the build-dir are installed to. This is a cache variable initialized to ${CMAKE_INSTALL_PREFIX}/share/dune/wheelhouse. The problem is that if CMAKE_INSTALL_PREFIX is set after the first cmake run, this variable is not updated since cached. The effect is, that wheel files will be installed in a read-only system location, leading to a cmake error, see #339 (closed). The MR solves this issue by creating a meaningless value first and setting it afterwards.

Closes #339 (closed).