Explicitly set sprockets as asset pipeline

Propshaft is default from Rails 8. See rails/rails#51799

spec/support/unit/rails_application.rb

@@ -90,7 +90,7 @@ def rails_new
     end
 
     def rails_new_command
-      "bundle exec rails new #{fs.project_directory} --database=#{database.adapter_name} --skip-bundle --skip-javascript --no-rc --skip-bootsnap"
+      "bundle exec rails new #{fs.project_directory} --database=#{database.adapter_name} --asset-pipeline=sprockets --skip-bundle --skip-javascript --no-rc --skip-bootsnap"
     end
 
     def fix_available_locales_warning