[FIX] sale_margin: recompute purchase_price on product_id_change.

When creating sale order lines from the matrix (sale_product_matrix),
the purchase_price weren't computed

No cleaner change has been found to ensure the cost is computed.

Triggering all onchanges from the server side is quite tricky and dirty
to do...

closes odoo/odoo#40201

Signed-off-by: Damien Bouvy (dbo) <dbo@odoo.com>