[FIX] mrp: use work center's TZ to plan operations

When planning a Manufacturing Order, if an operation takes place in work
center with a different time zone, the computed date may be incorrect
(the start date may be outside the working hours)

To reproduce the error:
(Use demo data. Current timezone: Europe/Brussels)
1. In Settings, enable "Work Orders"
2. Open an existing Work Center
3. Click on "Standard 40 hours/week"
4. Set Timezone to "Asia/Bangkok"
    - Note that all slots are between 8:00-12:00 and 13:00-17:00
5. Create two storable products P_compo and P_finished
6. Create a Bill of Materials BM:
    - Product: P_finished
    - BoM Type: Manufacture this product
    - Components: 1 x P_compo
    - Operations: 3 x Operation with existing work centers
7. Create a Manufacturing Order:
    - Bill of Material: BM
    - Quantity: 100
8. Save, Confirm, Plan

Error: (it depends on the time the test is done) The 'Scheduled Start
Date' of the second operation is incorrect. Adding the time zone
difference gives a time that is outside the work center's timetable
(i.e., outside 8:00-12:00 and 13:00-17:00).

The computations do not consider the time zone of the work center.

OPW-2393330

closes odoo/odoo#70841

Related: odoo/enterprise#18524
Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>