Fitenvironment is broken for nnpdfcpp fits

Created by: Zaharid

https://github.com/NNPDF/nnpdf/pull/1328 added code like https://github.com/NNPDF/nnpdf/blame/f2ad3ea60cec9c93b958b5ec77387d82142e8575/validphys2/src/validphys/config.py#L274 which is not valid for nnpdfcpp fits since they don't have the corresponding keys. While we have #1405 (closed) for now it would be good to be on top of what works and what doesn't (but not sure about what to do here).