[FIX] purchase_stock: propagate supplier in multi steps

Backport of: https://github.com/odoo/odoo/commit/1e550be1cc968940dd5c263160b8cc38e9d4e72b



Steps to reproduce the bug:
- Enable Multi-step Routes in inventory settings
- Go to Inventory > Warehouses> San francisco > Set Incoming Shipments
   to 2 steps
- Create a Test Product:
    - Route: Buy
    -  vendors Azure Interior and Gemini Furniture in this exact order
- Create a Replenishment Rule for the “Test Product”:
    - route: Buy
    - Vendor: “Gemini Furniture”
    - min and max quantity to 0
    - To Order: 1
    --> Click Order Once

Problem:
Check the purchase order, it will have “Azure Interior” as the vendor
and not “Gemini Furniture”

opw-3099958

closes odoo/odoo#110449

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