Do not install HepMC3 examples
HepMC3's CMake installs examples by default. This is problematic as the installed directories are detected as packages by the old Gaudi CMake (see https://gitlab.cern.ch/lhcb-core/LbDevTools/blob/eec8bdba6fd34245524559197b0dfbd0550cd470/LbDevTools/data/cmake/GaudiProjectConfig.cmake#L364).
/cc @clemenci