DEV: Check Zeitwerk eager loading in GitHub CI (#20699)

In production, `eager_load=true`. This sometimes leads to boot errors which are not present in dev/test environments. Running `zeitwerk:check` in CI will help us to pick up on any errors early.

This commit also introduces a `DISCOURSE_ZEITWERK_EAGER_LOAD` environment variable to make it easier to toggle the behaviour when developing locally.

config/environments/development.rb

@@ -9,7 +9,7 @@ Discourse::Application.configure do
   config.cache_classes = false
   config.file_watcher = ActiveSupport::EventedFileUpdateChecker
 
-  config.eager_load = false
+  config.eager_load = ENV["DISCOURSE_ZEITWERK_EAGER_LOAD"] == "1"
 
   # Use the schema_cache.yml file generated during db:migrate (via db:schema:cache:dump)
   config.active_record.use_schema_cache_dump = true

config/environments/test.rb

@@ -44,7 +44,7 @@ Discourse::Application.configure do
   config.assets.compile = true
   config.assets.digest = false
 
-  config.eager_load = false
+  config.eager_load = ENV["DISCOURSE_ZEITWERK_EAGER_LOAD"] == "1"
 
   if ENV["RAILS_ENABLE_TEST_LOG"]
     config.logger = Logger.new(STDOUT)