Use token issued from Grader Service for requests

[meffmadd]

Is your feature request related to a problem? Please describe.
With Grader Service becoming an IDP (TU-Wien-dataLAB/grader-service#108), we need to use the token issued by the service for requests.

Describe the solution you'd like
Pass the environment variable when spawning and use it in the header instead of the JupyterHub API token.