Fix another CIfail ⚡ 🔥 Deleting RAILS_ENV from ENV doesn't produce...

Fix another CIfail ⚡ 🔥 Deleting RAILS_ENV from ENV doesn't produce the desired result if RACK_ENV is still defined. And in any case it's better to be explicit here, so just set RALS_ENV=development.

railties/test/isolation/abstract_unit.rb

@@ -91,7 +91,8 @@ def assert_body(expected, resp)
   module Generation
     # Build an application by invoking the generator and going through the whole stack.
     def build_app(options = {})
-      @prev_rails_env = ENV.delete('RAILS_ENV')
+      @prev_rails_env = ENV['RAILS_ENV']
+      ENV['RAILS_ENV'] = 'development'
 
       FileUtils.rm_rf(app_path)
       FileUtils.cp_r(tmp_path('app_template'), app_path)