[FIX] mrp: allow backorder immediate production for SN product

Steps to reproduce:
1. Create Manufacturing Order with quantity >1 of Serial Tracked product
2. Press Mark done. (One Product will be immediated produced with a new
   SN auto-created)
3. Create Backorder
4. Press Mark done on the backorder

Expected Result:
Backorder will also be able to immediate produce and auto-create a new SN

Actual Result:
Usererror because env is still referring to original MO and tries to
produce the same SN again.

closes odoo/odoo#79123

Fixes: odoo/odoo#78134
Signed-off-by: Arnold Moyaux <arm@odoo.com>