Validate path for icu-i18n

Add to preflight check to verify that icu-i18n is available in the system path.

Migrated from: #594 (closed)

This looks like it's already in the docs https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/troubleshooting.md#elasticsearch

  1. Building the elasticsearch-indexer failed due to the icu4c issue linked in the troubleshooting. I had to set export PKG_CONFIG_PATH="/usr/local/opt/icu4c/lib/pkgconfig:$PKG_CONFIG_PATH".
Edited by Kyle Wiebers