Spree::Variant#set_position conflicts with acts_as_list

[tkimi]

When creating a Variant, acts_as_list and set_position callback increment position.
So I think this commit needs to be picked.

Solidus Version:
4.3.0.dev

To Reproduce

$ RAILS_ENV=test bin/rails c
irb(main):001> require 'factory_bot_rails'
irb(main):002> variant = FactoryBot.create :variant
...
irb(main):003> variant.product.master.position
=> 1
irb(main):004> variant.position
=> 3
irb(main):005> multi_variant = FactoryBot.create :variant, product: variant.product
...
irb(main):006> multi_variant.position
=> 5

Current behavior
Not increase by 1

Expected behavior
Increase by 1