[WIP] fish: work around default path_helper shuffling PATH

[thefloweringash]

Wrap the main fish config to undo its changes to PATH in
path_helper. It puts injected paths before other entries, which causes
nix-specific paths to appear after system paths.

Fixes #122.

[thefloweringash]

It's a little disappointing to clobber the path_helper mechanism, but this is consistent with other shells under nix-darwin.

[thefloweringash]

I think the nixpkgs initialization is inhibited in child shells, which will break this approach.

[LnL7]

I'm not familiar with fish, how does this compare to the nixos module? https://github.com/NixOS/nixpkgs/blob/c941577dcb68ad413853ae744a458555b3209c21/nixos/modules/programs/fish.nix#L109-L170

[thefloweringash]

The short answer is: this doesn't work. I don't know if I can fix it yet. Also I'm not familiar with fish either, first time looking at it trying to fix this!

The longer answer: it tries to work exactly the same as NixOS, but fish internally rewrites the PATH on every shell, so we're trying to undo that.

NixOS sets up environment once per shell tree, ignoring what kind of shell it is. This is a coarse but effective approach that means running commands directly from ssh work. Most shells are either launched with a clean environment for NixOS to setup, or launched as non-login shells.

The macOS path_helper isn't a problem for bash for two reasons:

1. it only runs for login shells
2. it runs before the nix-darwin module replaces the PATH.

(1) allows child shells to inherit paths, since they're typically non-login.
(2) allows nix-darwin to have the final say in the value of PATH

Neither of these are currently true in fish in nix-darwin. (1) was true, but was reverted, and (2) is a consequence of nix injecting the environment early in fish's lifecycle. I'm not sure which problems the early injection fixes. Maybe we could do environment configuration at the top of /etc/fish/config.fish, so as to run after the path_helper?

[thefloweringash]

I don't think this is the right approach. Sorry for noise.