The use of the CMake variable DUNE_PYTHON_WHEELHOUSE is inconsistent

The following discussion from !1417 should be addressed:

• @simon.praetorius started a discussion:

  • @simon.praetorius: This variable was a non-cache variable by reason, see !1299 (merged). Maybe there is a better alternative.

  • [...]

  • @santiago.ospina: The problem is that we are modifying two different variables here. It "works" as expected because local variables take precedence over the cache. In super builds, the scope of this local variable is not seen by other directories and the wheelhouse option ends up expanding with the cache into something like --find-links=file://unset.