Fix for failing dataflow build

[jackal1-66]

Hybrid generator includes Pythia8, so when the dataflow build happens, without this fix, the Pythia8 header is loaded. It's not found due to the nature of the dataflow build. This fix prevents the Hybrid generator loading if Pythia8 is not found and it includes also HepMC3, given that a similar issue might arise with that generator as well in the future.

[github-actions]

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[ktf]

Did you test it locally? If yes, I will go ahead and merge it since this is currently broken.

[jackal1-66]

Let's wait for the CI results, I rebuilt O2sim with my slc9 system and it works, but I can't test the dataflow build on cs8.

[jackal1-66]

build/O2/o2_slc9 failure is related to o2sim_checksimkinematics_G3, so unrelated to the purpose of this PR

[jackal1-66]

I managed to reproduce the error locally and I ultimately fixed the issue. If there are no other differences between my slc9 build and the cs8 one, then I expect the next dataflow test to be successful