nixos/roundcube: don't use the default and insecure des_key

The php installer creates a random one, but we bypass it, so we have
to create one ourselves.

This should be backward compatible as encryption is used for session
cookies only: users at the time of the upgrade will be logged out but
nothing more.

https://github.com/roundcube/roundcubemail/blob/259b7fa0650fea9320b38cb17c4e80497acae7a3/config/config.inc.php.sample#L73