Use fish as nix shell. Works with other shells too

default.nix

{ system, nixpkgs ? <nixpkgs> }:
let pkgs = import nixpkgs {inherit system;};
in pkgs.stdenvNoCC.mkDerivation {
  name = "fshell";

  script = pkgs.substituteAll {
    src = ./fshell.bash;
    bashInteractive = pkgs.bashInteractive;
  };

  phases = ["installPhase"];

  installPhase = "mkdir -p $out/bin; install $script $out/bin/fshell";
}

flake.nix

{
  description = "A very basic flake";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
  };

  outputs = { self, nixpkgs }: {

    packages.aarch64-darwin.fshell = import ./default.nix { system = "aarch64-darwin"; inherit nixpkgs; };

    packages.aarch64-darwin.default = self.packages.aarch64-darwin.fshell;

  };
}

fshell.bash

#!@bashInteractive@/bin/bash

# Taken from: https://ianthehenry.com/posts/how-to-learn-nix/nix-zshell/
# Modified to use fish, and modified derivation to be able to be installed with flakes
# To use: EXPORT NIX_BUILD_SHELL=fshell

# when using nix-shell, we will be invoked like this:
#
#     /path/to/result --rcfile /path/to/file
#
# When using nix-shell -i, we will be invoked like this:
#
#     /path/to/result /path/to/file
#
# We use the presence of --rcfile to decide if we're supposed
# to be run interactively or not.

if [[ "$1" = "--rcfile" ]]; then
  save_shell="$SHELL"
  rcfile="$2"
  source "$rcfile"

  export SHELL="$save_shell"
  exec $(which fish)
else
  exec @bashInteractive@/bin/bash "$@"
fi