Ensure we run installation Rake tasks in a clean env in TestEnv

If we call `system('rake', 'taks_name')`, `ENV['RUBYOPT']` is set to
`'-rbundler/setup'` but some tasks (e.g. `gitlab:gitaly:install` need
a clean env since they install their own Gem bundle.

Signed-off-by: Rémy Coutable <remy@rymai.me>