ci: always test examples with fixed global seed
This should prevent random failures of the examples job in CI like e.g. https://gitlab.cern.ch/allpix-squared/allpix-squared/-/jobs/22345082.
This particular failure was caused by the line (WARNING) [F:DepositionGeant4] No charges deposited
. Apparently just setting the core PRNG is not enough, I guess because that one is not being used in Geant4?