Allow "pure" shells by removing non-Nix elements in PATH

I use `nix develop --unset PATH` locally to make sure all my dependencies are installed through Nix and I'm not using anything already installed on my system (e.g. via `apt`).

It would be great if `nix-develop` could do the same, or perhaps it's configurable in some other way through `flake.nix`?

Thank you for this action, it really got me up and running in no time at all! Super useful.