[FIX] stock: speedup _get_inventory_lines_values

When an inventory has lots of non-zero lines,
i.e. lines with non-exhausted products,
action_start is currently quite slow
because of the browse inside the for loop
of _get_inventory_lines_values.

This commit speeds it up by moving
the browse outside of the for loop.

closes odoo/odoo#80255

Signed-off-by: William Henrotin (whe) <whe@odoo.com>