Fix flake compat

[berberman]

Closes #1846

We should enter devShell rather than the shell of defaultPackage.

[ErnestKz]

Hey, just some questions to help me learn:

In theory would keeping the file named default.nix still work? What does the .shellNix do on the flake import? Is the .shellNix something that's defined outside of this repo?

Also thank you for fixing this.

[berberman]

In theory would keeping the file named default.nix still work?

Yes, nix-shell will use default.nix if there is no shell.nix.

What does the .shellNix do on the flake import? Is the .shellNix something that's defined outside of this repo?

It's defined in https://github.com/edolstra/flake-compat/blob/master/default.nix, which basically allows you use flakes with the legacy nix commands (nix-build and nix-shell). You can find more details in "Using flakes project from a legacy Nix" of https://nixos.wiki/wiki/Flakes.

[Ailrun]

Awesome, thanks!