build: make icu download path customizable

This makes it easier to store icu tarballs outside of the node.js
directory which is useful in our CI where git directories are
scrubbed between runs.

PR-URL: https://github.com/nodejs/node/pull/3200


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Rod Vagg <rod@vagg.org>