[FIX] mrp: set default dest loc of by-products

To reproduce the issue:
1. In Settings, enable
   - Storage Locations
   - By-Products
2. Edit the warehouse:
   - Manufacturing: 3 steps
3. Create four products
4. Create a BoM:
   - Product: P01
   - Components: P02
   - By-products: P03
5. Create a MO
6. Add P04 to by-products
7. Confirm the MO
8. Open the detailed operations of P04

Error: When trying to add a new SML for P04, the default destination
location is `WH/Pre-Production`, it should be `WH/Post-Production`

OPW-3340274

closes odoo/odoo#127358

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