Modify Geant4 recipe to make possible to define an external directory for data sets, plus clean-up:

[ihrivnac]

• This PR restores back ca28635
• Fix setting Geant4 data sets in build environment (failing in the previous version):
  • Removed setting data sets in env: section in geant4.sh
  • Added setting data sets environment via geant4-config in recipes with Geant4 tests
    (aliroot-guntest.sh, o2.sh)

[ihrivnac]

@ktf, @sawenzel , can you approve ? Thank you.

[ihrivnac]

@ktf , can you merge ? Thank you.

[sawenzel]

@ktf: Shouldn't simulation experts be able to merge this as per CODEOWNERS? The button is not activated.

[ktf]

@sawenzel the issue was that since o2.sh and aliroot-gun.sh were modified, it required a review from alisw@infrastructure-admins. I added alisw/pdp-experts for all the "mostly PDP" things, so next time your review should be enough. Of course I could have added you to alisw@infrastructure-admin (or you can add yourself, since you should be a global admin), however I think it's better if we keep infrastructure-admin separate, so that we have some better chance to notice new recipes / recipes without a proper owner.