[FIX] account_edi_facturx: Set gross_price_total_unit value properly

Previously (since https://github.com/odoo/odoo/commit/649e0f2d1aefed585c3cf2b0c96ff6004280de97

) it was only set when the invoice had taxes, and it was re-defined as many times as taxes.

closes odoo/odoo#106862

Signed-off-by: William André (wan) <wan@odoo.com>