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.

They are both as powerful, but buildEnv is treacherous: if you pass a
package which depends on another python (for example the one of unstable
when you are on stable) it will be *silently* dropped, leading to hair
pulling.

Use case:
override neovim from unstable, but still keep stable's pythonPackages.

A function of the same signature as the argument of python.withPackages

Fix neovim PYTHONPATH handling

wine{Unstable,Staging}: 3.13 -> 3.14

- it can't be built as a module
- it's not set as the default yet
- for discussion see NixOS/nixpkgs#42330

(cherry picked from commit 093eb27a9e5793343f6506d063debb3d11794ac1)

(cherry picked from commit aa52aa68a33313e4e209872a0422622ae65116cc)

(cherry picked from commit b2f042dfd8edf67d3096ff3294ff9050890ecfdd)