Draft: Speed up projectenv.sh by removing duplicates at the source

Two changes in separate commits, please review them one at a time to make it clearer.

• Speed up projectenv.sh generation and execution : moved to !3824 (merged)
  • Improve projectenv.sh generation for super-project builds by populating extra_commands only once (needs cmake 3.19).
  • Use a faster shell-based duplicate removal.
• Speed up projectenv.sh by removing duplicates at the source. This is more controversial, it might interfere with !3159 (merged), but since I don't really understand it, @clemenci I need your help. In particular, it would be good to note down what is the reason to "override GaudiToolbox internal macro to use lhcb_env".