[FIX] hr: search on employee_id field raise bad query

Problem
-------
A user with no read access on private employee (hr.employee) model
will get a bad query error if he search on employee_id on res.users

The new domain evaluation system generate query like this
SELECT "hr_employee"."user_id" FROM "hr_employee_public"
because the _search of hr.employee was not returning the search result
from the right table

Solution
--------
Return a search result from the right table

closes odoo/odoo#62944

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>