Add actor to `ui_for_organizations` feature flag check

What does this MR do and why?

Related to #417875 (closed)

In !126225 (merged) we setup the Organizations section in the Your work sidebar but we forgot to add an actor to the ui_for_organizations feature flag check. Because of this the Organizations menu does not show in the sidebar on gitlab.com because we have it enabled with user actors. There is another ui_for_organizations feature flag check in app/controllers/organizations/application_controller.rb#L5 that does have an actor. This MR adds the current user as the actor for the feature flag check in the sidebar.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. bin/rails console
2. user = User.find_by_username(<your username>)
3. Feature.enable(:ui_for_organizations, user)
4. Sign into GDK

MR acceptance checklist

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