Python version used by node

[FRidh]

I'm working on migrating Nixpkgs to Python 3 as default. Not all packages support 3.x yet, and in those cases python2 needs to be explicitly passed in. I will make the changes in the Nixpkgs repo, even on the generated files, but you will need to explicitly use python2 here. Furthermore, I will add passthru.python to nodejs so I recommend nodeEnv uses that instead. That way we can guarantee the same version of Python is used.

Branch: https://github.com/NixOS/nixpkgs/tree/python3
PR: NixOS/nixpkgs#19626
Issue: NixOS/nixpkgs#18185

[svanderburg]

The only solution I have in mind is that we must make the python version used configurable so that we can retain compatibility with both the master and latest stable branches.

[svanderburg]

Since rev aabfd15, we now explicitly refer to python2