build: don't create duplicate bootstrap-tools targets

[kevans]

Summary

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.

[imp]

This is obscure enough we may want to have a comment..

[imp]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=289997

[dhw]

The suggested change was effective in my testing.

[kevans]

In D52904#1208870, @imp wrote:

This is obscure enough we may want to have a comment..

I'll add a note