[FIX] website: fix primary inherit view validation when tree is COW'd

Before this commit, it was not possible to install website_forum if the 'Main
Frontent Layout' had been COW'd.

When loading Forum Layout view which is a primary child view, it would crash
during view validation, not finding the element to xpath.

Triggering COW instead of copying fix the issue.

opw-2036119

closes odoo/odoo#35138

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>