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

lib/autospec/simple_runner.rb

@@ -32,7 +32,7 @@ module Autospec
       end
 
       # launch rspec
-      Dir.chdir(Rails.root) do
+      Dir.chdir(Rails.root) do # rubocop:disable DiscourseCops/NoChdir because this is not part of the app
         env = { "RAILS_ENV" => "test" }
         if specs.split(' ').any? { |s| s =~ /^(.\/)?plugins/ }
           env["LOAD_PLUGINS"] = "1"