[FIX] core: avoid access error when flushing

This patch guarantees that computed fields with `compute_sudo=True` do
not cause access errors when fetched from database.

This fixes a non-deterministic access error upon `commit`, where fields
are recomputed and flushed with some random environment.  The method
`recompute` performs its duty by accessing the fields to recompute.  The
compute method of a field is called but does not assign it.  As the
field is not in cache, its value is read from database.  However, as the
latter is not done in superuser mode, it may cause some access error.

closes odoo/odoo#61820

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>