[FIX] mrp: unit_factor computation

Updating the quantity to produce in a production order will recompute
each raw move's unit factor. The issue was this computation did not
take care of the previously product quantity. The unit factor was wrong
and so the next created workorder lines get the wrong quantity.

Example:
  - 1 components for 1 finished product (unit_factor = 1)
  - Create a production for 2 finished product -> quantity to consume = 2
  - Produce 1 then change quantity to produce to 3 -> quantity to consume = 3
    and quantity done = 1 but unit factor became 1.5

closes odoo/odoo#40932

X-original-commit: 71ea4d4488a085030ae07ca7e85cafc21f06af3b
Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>