fix(poulpy): better integration for nushell and restish
What
Allow user to use his own nushell and restish binaries
Why
Because on some OS (NixOs for instance), nushell do not work if not installed with nix.
So we need this fallback to make sure the user can use poulpy if he install those binaries himself
How
Download and use binaries only if they are not already in the path