neovim wrapper: do not unset PYTHONPATH

This solves the following bug:
opening neovim in nix-shell -p pythonPackages.numpy does not enable to
run successfully
:!python -c "import numpy"
because the PYTHONPATH is wiped by the neovim wrapper.

This wiping is necessary for the python providers, though, otherwise a
python2 nix-shell will make the python3 provider read python2 files.

We wrap the providers only, instead of neovim as whole.