Use the correct CMake variable name for PYTHIA8_VERSION

-- Found Pythia8: /usr/include (found suitable version "8.306", minimum required is "8.220") -- HepMC3 test: Pythia8 package found. /usr/lib64/libpythia8.so 8.306 -- HepMC3 test: WARNING: Will use deprecated built-in HepMC3 interface for Pythia8.

With this change the warning about using the deprecated built-in version disappears.