Assign attribute_lists in a single write

[stefanlew-tractive]

EvaluatorClassDefiner#evaluator_class writes attribute_lists, then reads it back on the next
line:

klass.attribute_lists ||= []
klass.attribute_lists += [@attributes]

attribute_lists is a class_attribute, so the write goes through ActiveSupport's generated writer
and the second statement re-reads through the generated reader. If that read does not observe the
write, + is called on nil:

NoMethodError: undefined method '+' for nil
  lib/factory_bot/evaluator_class_definer.rb:16:in 'block in evaluator_class'

Reproduction against 6.6.0:

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"
  gem "factory_bot", "~> 6.6"
end

require "factory_bot"
require "minitest/autorun"

class EvaluatorClassDefinerTest < Minitest::Test
  def test_evaluator_class_does_not_read_attribute_lists_back
    parent_class = Class.new(FactoryBot::Evaluator)
    parent_class.define_singleton_method(:attribute_lists) { nil }
    parent_class.define_singleton_method(:attribute_lists=) { |value| value }

    FactoryBot::EvaluatorClassDefiner.new([], parent_class).evaluator_class
  end
end

This collapses the pair into a single read-modify-write, so the value is never read back after being
written. Behaviour is unchanged: a fresh subclass still starts from an empty list, and a child
evaluator still accumulates its parent's lists. The added spec fails on main and passes with the
change.