Fix broken variant weight assignment in spec

OrderManagement::Stock::Packer builds a package with all the items
     Failure/Error: expect(package.weight).to be_positive
       expected `0.0 (#<BigDecimal:7f156c99fda8,'0.0',9(18)>).positive?` to be truthy, got false
     # ./engines/order_management/spec/services/order_management/stock/packer_spec.rb:19:in `block (2 levels) in <module:Stock>'

engines/order_management/spec/services/order_management/stock/packer_spec.rb

@@ -10,7 +10,7 @@ module Stock
 
       subject { Packer.new(stock_location, order) }
 
-      before { order.line_items.first.variant.update(weight: 1) }
+      before { order.line_items.first.variant.weight = 1 }
 
       it 'builds a package with all the items' do
         package = subject.package