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

[dotdoom]

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.