[FIX] mrp: workorder cost not included in price

When auto-fill the duration of workorders in immediate production, the
workorder cost is not included in the price of the product.
It happens becuase we calculate the price in _post_inventory but we set
the duration after _post_inventory. To fix, we set the duration of the
workorders in before _post_inventory.

closes odoo/odoo#111804

Signed-off-by: William Henrotin (whe) <whe@odoo.com>