build: only emit download ICU warnings once

The check that the user specified `icu` in `--download` only needs to be
done once and not for each entry in `tools/icu/current_ver.dep`.

Fixes: https://github.com/nodejs/node/issues/26860

PR-URL: https://github.com/nodejs/node/pull/27031


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>