That's how Get Current User is designed to work I believe.
If your flow is form based, and you're integrating with PM, you can just use the PM Login component... see this thread:
https://www-secure.symantec.com/connect/forums/how-obtain-sessionid-using-workflow
If you're not using PM, you can try the Get HTTP Request Value component, but you won't get good results if your WF is running under anonymous creds (would have to be windows auth to work). See this thread:
https://www-secure.symantec.com/connect/forums/401-unauthorized-using-http-get-invoke-workflow-task