Add `xonsh-dev-env` to enter a nix develop-esque shell with Xonsh

[Lunaphied]

Launching Xonsh as a command under nix develop doesn't properly inherit the bash functions which is rather annoying if you need to run Nix unpack and configure phases manually while working on a project.

This adds an alias that utilizes source-bash and nix print-dev-env and temporary files to create a usable environment, it also fixes a bug that affects nix develop on it's own as well ($SHELL must be bash)