[Cookbook] Added note to deployment tools about exporting SYMFONY_ENV

Fixes an issue (https://github.com/symfony/symfony-standard/issues/662) on symfony-standard.
Why not be even more specific? Something like:

```sphinx
If you get a "class not found" error during this step, you may need to
run ``export SYMFONY_ENV=prod`` before running this command so that
the ``post-install-cmd`` scripts run in the ``prod`` environment.
```