fish: add patch to fix $PATH for nix-darwin

[emilazy]

See the comment for details. This is adapted from my nix-darwin configuration.

cc @Enzime

cc @RossComputerGuy – this is technically a breaking change, but only when a variable is set that is only set by nix-darwin, and as a nix-darwin maintainer I affirm that it is the kind of bug‐fixing breaking change that we really want; this has been a very long‐standing issue with nix-darwin and it would be sad to tell users of stable branches they have to wait 6 months for a fix.

cc @lilyball who I think uses an alternative workaround

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[emilazy]

Moved the conditional to a more appropriate place.

[cole-h]

LGTM.