Fix build on systems where Pythia is not available

The build process was broken on systems that did not have Pythia8.

The vertical_EAS example requires Pythia however the current CMake build process would still try to compile the example code even when Pythia was unavailable (resulting in a compile error). This MR explicitly adds the dependence on Pythia8 into the CMake configuration for the vertical air shower example (using an existing if (Pythia8-FOUND)-block). This has been tested on systems with and without Pythia.

The README suggests that Pythia is an optional component - is the plan for Pythia to remain optional? If so, it would probably be good to create at least one CI task to build CORSIKA8 on a system without Pythia.