[FIX] sale_mrp: filter variant BOMs for anglo_saxon price_unit

When invoicing a product with multiple variants and with anglo_saxon_accounting, the COGS would be the wrong amount.
The function _stock_account_get_anglo_saxon_price_unit() was selecting the first BOM among all variant BOMs.
Using the variant_bom_ids when existing prevent this situation.

OPW-2662863

closes odoo/odoo#78595

X-original-commit: cab1b34d
Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>