pythia8: fix configure args

[jmcarcell]

Adding a string to a list in python with += adds each character as one element of the list instead of adding the whole string as a single element so the configure args look like this:

--with-hepmc2=/cvmfs/sw-nightlies.hsf.org/key4hep/releases/2023-10-06-paestum/x86_64-ubuntu22.04-g
cc11.3.0-opt/hepmc/2.06.11-3hwln5 - - w i t h o u t - m g 5 m e s --without-hepmc3 - - w i t h o u
 t - f a s t j e t 3 --without-evtgen --without-root --without-rivet --without-python --without-op
enmp --without-mpich --without-hdf5

[ChristianTackeGSI]

I have an idea, could you look into it?

If this does not work, I am happily approving this, because this is obviously wrong the current way.

[tldahlgren]

@jmcarcell ping me when you're ready for this PR to be merged.

[jmcarcell]

@tldahlgren It's ready for merging