Follow-up to #40143.

Rails Rake tasks are loaded for each new `Rake::Application` instance
via `Rails.application.load_tasks`.  However, under the hood,
`Rails.application.load_tasks` used `require` instead of `load`, which
caused Rails tasks to be loaded for only the first `Rake::Application`
instance.

This commit changes the relevant `require` to `load`.

Fixes #40184.