[FIX] base: ensure all attachments are in sudo

>>> u1 = self.sudo(False).browse(1)
>>> u2 = self.sudo().browse(2)
>>> (u1 + u2).env.su
False
>>> (u2 + u1).env.su
True

Ensure all attachments are always in sudo

Before this commit, a portal user could not go in debug asset

Introduced at 3a98996e

closes odoo/odoo#80754

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>