[17.0][FIX] sale_fixed_discount: fix portal template

[SilvioC2C]

Fixes portal template incorrect rendering of discounted unit prices.

SO lines info

From the SO form view's order_line tree view (using quantity as sequence to ease comparison w/ the template reports below):

NB: the fact that the "Disc. %" on the second line is 0 even though the "Discount (Fixed)" is 0.10 is not an error: since field discount is computed but not readonly, there are cases in which field discount could be assigned a value which is different than the one assigned by the compute method (for example, when importing the SO through the Odoo file importer).

Portal report before the fix

🔴 line 1: the discounted unit price is 1.00 instead of 0.90 (10% applied on 1.00)
🔴 line 2: the discounted unit price is not shown at all
🟢 line 3: the discounted unit price is shown correctly (3.00 w/ 0.30 or 10% discount = 2.70)
🟢 line 4: no discount applied

Portal report after the fix

🟢 line 1: the discounted unit price is shown correctly (1.00 w/ 10% discount = 0.90)
🟢 line 2: the discounted unit price is shown correctly (2.00 w/ 0.10 discount = 1.90)
🟢 line 3: the discounted unit price is shown correctly (3.00 w/ 0.30 or 10% discount = 2.70)
🟢 line 4: no discount applied