python: allow venv to be used in recursive devenv

[bobvanderlinden]

Currently whenever you have a project structure like:

.envrc
devenv.nix
subproject/.envrc
subproject/devenv.nix

And you navigate into subproject, you'll get both environment variables from the root and from subproject. This works surprisingly well, however venv doesn't.

It seems as if DEVENV_ROOT from the subproject is being concatenated to DEVENV_ROOT of the parent project by envrc.

To circumvent this venv should stop relying on the runtime DEVENV_ROOT and DEVENV_PROFILE and start relying on the variable from Nix.