Fix flake for nixpkgs on darwin

[TomMD]

Three issues addressed.

1. On darwin we need SystemConfig as a native build input.
2. Darwin doesn't have libudev-zero
3. Most darwin users will have the default zsh shell. When invoking nix develop they'll get a bash shell. The shell script setting up autocompletion of commands needs to not use the SHELL variable because that will be zsh and not the correct value of bash. See below for nix develop with this PR (without this PR the first result is zsh and thus the completion script fails).

$ echo $shell
bash
$ echo $SHELL
/bin/zsh
$ echo $0
bash

[leighmcculloch]

I defer to @Ifropc that knows more about flakes and nix than I do, but there is one thing being removed as part of this PR that I think is unrelated, but good. And there are two other things I think we should also remove.

It would be ideal if we could remove those unrelated three things in a separate PR though.

[Ifropc]

Thanks for the fix @TomMD