[FIX] stock: more flexible xpath

If you have stock installed before 0322f6e5 and try to install stock_account
today, the installation fails.

Should not rely on a field added after the stable freeze

Fixes #21823
opw-802767

addons/stock_account/views/stock_account_views.xml

@@ -43,9 +43,9 @@
             <field name="inherit_id" ref="stock.view_stock_return_picking_form"/>
             <field name="model">stock.return.picking</field>
             <field name="arch" type="xml">
-                <field name="uom_id" position="after">
+                <xpath expr="//field[@name='product_return_moves']/tree" position="inside">
                     <field name="to_refund"/>
-                </field>
+                </xpath>
             </field>
         </record>