Move gaussino slots to LCG 99 & fixes in the old configuration

This MR introduces the following changes:

• Move lhcb-gaussino and lhcb-gaussino-fastsim slots to LCG 99.
• Fix the old configuration of the environmental variables.
  • slot.env = cmakeEnv was added on top of already appended changes in slot.env. It's been moved before all slot.env.append. Once we're here, since the slots worked already without those changes (most of them follow, I think, the configuration of the lhcb-dd4hep slot), are they really up-to-date?
  • appending slot.env resulted in fact in appending cmakeEnv which is common also for other gauss slots. I've added a list(cmakeEnv) to ensure these changes are only for lhcb-gaussino and lhcb-gaussino-fastsim

@clemenci @gcorti