DataSink to different files when switching between Environment

You can add a Project Level custom property to your project, and then this property can be set per environment via the Environments editor.

Then in your DataSink test step you can refer to the project property via a property expansion. The easiest way to do this is to right click in the "File" text box in the DataSink test step and use the "Get Data" menu option to select your newly created project property.