Explicitly specify pythia6 dependency for AliPythia6
Explicitly specify pythia6 dependency for AliPythia6

Previously, analysis trains relied on the pythia6 library being pre-loaded, but
that pre-loading is a ROOT5-ism. Instead, make sure libAliPythia6 is linked
properly against pythia6.

This should solve the `libAliPythia6.so: undefined symbol: pyqpar_` error seen
by analysers.

This seems to fix the problem in the one instance I reproduced locally based on
the run at:
http://alitrain.cern.ch/train-workdir/PWGGA/GA_pp_MC/3980_20221018-2243/test/__BASELINE__

While I did not reproduce the full run with all *.root files, I managed to
reproduce the crash (without this patch), which did not occur with this patch
applied.

alisw/AliRoot,alisw/AliRoot,alisw/AliRoot,alisw/AliRoot,alisw/AliRoot,alisw/AliRoot,alisw/AliRoot,alisw/AliRoot