[FIX] account: set main attachment only for posted invoices

When printing a draft invoice, it raises an error.

The error comes from fix #65320: the latter registers the printed
invoice as main attachment, but this needs to be done only with posted
invoices.

OPW-2427247

closes odoo/odoo#66516

Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>