If we're going to do this, we need to do it for the specific case we want to change, rather than all uses of Plek and anything else depending on `GOVUK_APP_DOMAIN`.

What about adding some code like this to `whitehall_orgs.rb`:
```ruby
Plek.new(GOVUK_APP_DOMAIN || 'production.alphagov.co.uk').find('whitehall-admin')
```
That way, if you want to run it against Whitehall dev, you can explicitly set `GOVUK_APP_DOMAIN=dev.gov.uk`, but by default (and in production) it'll use GOVUK_APP_DOMAIN. We could additionally wrap it in ``if Rails.env.development? ... else`` for clarity and safety.