support both classical nix and flakes

[Janik-Haag]

{ pkgs }: means the shell.nix file expects to be called with an already initialized version of nixpkgs, which is not the case when using the nix-shell command. The flake.nix / nix develop avoids this by passing an already initialized version.
This change makes the shell.nix check if its caller is calling it with a pkgs attribute and if that's not the case it just imports pkgs from the nixpkgs channel.

[keegancsmith]

Thanks.

Side note: I feel silly now that before that we made ctags an overlay that we export in the flake. I don't expect anyone to consume it, we should just have that bit of code living in shell.nix.