Fix mailer previews loading

The previous attempt was failing because it was adding preview paths
before setting ActionMailer autoloads, causing Solidus previews folder
to be autoloaded by Zeitwerk without namespace, as the `lib` folder is
not a Zeitwerk root (and we cannot add it as one):

    Zeitwerk::NameError:
    expected file solidus/core/lib/spree/mailer_previews/carton_preview.rb
    to define constant CartonPreview, but didn't

Adding the previews path after setting the autoload fixes the issue
by having the same scenario we had before Rails 7.1, i.e. the file is
not autoloaded by Zeitwerk.

core/lib/spree/core/engine.rb

@@ -80,10 +80,10 @@ class Engine < ::Rails::Engine
         end
       end
 
-      initializer "spree.core.action_mailer.set_preview_path", before: "action_mailer.set_autoload_paths" do |app|
+      initializer "spree.core.action_mailer.set_preview_path", after: "action_mailer.set_autoload_paths" do |app|
         if Rails.gem_version >= Gem::Version.new('7.1.0')
-          solidus_preview_path = Spree::Core::Engine.root.join 'lib/spree/mailer_previews'
-          app.config.action_mailer.preview_paths << solidus_preview_path.to_s
+          solidus_preview_path = Spree::Core::Engine.root.join('lib/spree/mailer_previews')
+          ActionMailer::Base.preview_paths << solidus_preview_path.to_s
         end
       end