Basically the Nintex Actions execute in the context of the current user. If you want to execute them in another context, If you want your query to take into account all your items, you may have to use the Call WebService action with the Sharepoint service that query this list (and specify an elevated privileges account inside the action).
The webservice will then query the entire list, and a small XML query after will get you all the data.
thinking about it, you can also use the Action set action.
in the "common" part of the action you can execute the action set action within the context of the user who designed the workflow