build: don't create duplicate bootstrap-tools targets

The general problem is that we might have a somewhat complicated
dependency tree depending on bootstrap version requirements. We could
document when multiple bootstrap tools might have a shared dependency
and be careful to add them only once to the list, but that is a little
more fragile- particularly if we purge some bootstrap tools and need to
re-work the logic a little bit.

Just avoid redefining the build commands as we're iterating over the
list so that we can keep the actual requirements intact.

PR: 289997
Reported by: dhw, others
Reviewed by: imp
Fixes: 151bd3516b5 ("flua: support our flua modules in [...]")
Differential Revision: https://reviews.freebsd.org/D52904