[FIX] stock: _compute_qty_order should not take environement context

savepoint do a flush that throw the current context and then recompute
the stored field (with a random matching context from the transaction).
In the case of qty_to_order, it will call qty_forecast and then
_compute_quantities on product.product.

Currently it will call _compute_quantities with a mix of self.env.context
(from the transaction) and a context generated from the orderpoint (with
deterministic data). But sometimes in the context took from the
transaction it could remain some other key used by _compute_quantities
(e.g. warehouse_id). Since qty_to_order doens't depends from context
it should not use self.env.context and only use the deterministic
context from the stock.orderpoint

closes odoo/odoo#100108

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