[FIX] product: flush before invalidate in write override

Since [1] `computer_hdd_attribute_lines` is archived instead of unlinked in
`TestSaleProductAttributeValueConfig` which leads to indeterminism in tests due
to the missing `flush`.

Indeed `flush` is necessary before `invalidate_cache` since [2] if the data
that are pending being written must be saved to the database before the cache
invalidation.

[1] which highlighted the issue was merged after v13.0, but it still makes sense
to merge the current fix in the earliest version possible.

[1] 5361706e
[2] 9920f20e

closes odoo/odoo#43436

Signed-off-by: Christophe Monniez (moc) <moc@odoo.com>