This is an internal fix, not user facing. I noticed it while working on https://github.com/rails/rails/pull/48585.

The `async` adapter has an `immediate` option, which should only be used in tests. This option should tell the adapter to run jobs inline. This works correctly with `perform_later`, but it does not work with `enqueue_at`, which is what other internal mechanisms such as `retry_job` use.

This PR fixes this bug.