[FIX] mrp: fix bad workorder-move link

Workorder _action_confirm works in batch and is designed to
link workorders and moves per production.

Original code breaks the 'per production' constraint by linking
moves from all productions to workorders of the last one when
called on workorders from multiple productions.

closes odoo/odoo#86698

Task: 2797201
X-original-commit: b25f1e00
Signed-off-by: Arnold Moyaux <arm@odoo.com>