Readd $rails_rake_task = true

`$rake_rails_task` was removed in https://github.com/rails/rails/commit/e6747d87f3a061d153215715d56acbb0be20191f#commitcomment-1749296

Could we readd it or properly deprecate it like in https://github.com/rails/rails/commit/c3ca7ac09e960fa1287adc730e8ddc713e844c37 ?

/cc @josevalim