When specifying ENV var SYMFONY__SEARCH_ENGINE and installing eZ Platform/Enterprise like the following

export SYMFONY__SEARCH_ENGINE="solr"
php -d memory_limit=-1 app/console ezplatform:install --env prod clean

SEARCH_ENGINE is not forwarded to the subprocess that installs the index.
As result, the var will not be injected, as expected. Such mechanism is documented here:
http://symfony.com/doc/current/configuration/external_parameters.html

Tested on eZ Platform/Enterprise 1.4 and 1.5