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If you do a tty.die() in setup_run*, then the whole "spack
load" will fail. So updated the text and things a bit.

BUT: setup_run_* is also called during build. So we need to
set the env var during build. Do that in setup_build_*.

Users need to register to use fluka.
So output a warning telling them to register.
Make it possible to calm the warning using an environment
variable.

Add packages.