Speed up projectenv.sh generation and execution

Rosen Matevrequested to merge
rm-lhcb-env-1 into master
  • 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.

This cherry-picks the non-controversial part of !3458 (closed)

/cc @clemenci

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