DEV: Remove use of cd in the app (#8337)

`FileUtils.cd` and `Dir.chdir` cause the working directory to change for the entire process. We run sidekiq jobs, hijacked requests and deferred jobs in threads, which can make working directory changes have unintended side-effects.

- Add a rubocop rule to warn about usage of Dir.chdir and FileUtils.cd
- Added rubocop:disable for scripts used outside the app
- Refactored code using cd to use alternative methods
- Temporarily skipped the rubocop check for lib/backup_restore. This will require more complex refactoring, so I will create a separate PR for review

script/plugin-translations.rb

@@ -42,7 +42,7 @@ class PluginTxUpdater
     PLUGINS.each do |plugin_name|
       plugin_dir = File.join(@base_dir, plugin_name)
       Bundler.with_clean_env do
-        Dir.chdir(plugin_dir) do
+        Dir.chdir(plugin_dir) do # rubocop:disable DiscourseCops/NoChdir because this is not part of the app
           puts '', plugin_dir, '-' * 80, ''
 
           begin