[FIX] stock_account: AVCO with MRP

- Create a product P with:
  Costing Method       Average Price
  Inventory Valuation  Perpetual (automated)
- Create a component C with, costing 10.
- Create a BOM for P:
  1 Unit of C
- Create a MO for P, validate
  => a journal entry of 10 is created
- Modify the BOM for P:
  2 Units of C
- Create a MO for P, validate
  => a journal entry of 15 is created

The journal entry should be 20.

opw-1928342

closes odoo/odoo#30766