Simplify testing environment

[davidverweij]

Currently, due to the use of Github Actions, we rely on default values for environmental variables as set in config. Instead, we should not set defaults for environment variables, as this will then no longer act as a safe guard when deploying to production (as a missing .env will not be caught).

This is currently partially handed by returning a subset of settings in data_transfer/config.py, but could be improved on and extend to consumer/config.py.