nix.sh translated to fish, so the fish shell can be used as login shell

nix.fish

if test -n $HOME
  set NIX_LINK "$HOME/.nix-profile"

  # Set the default profile
  if test ! -L "$NIX_LINK"
    echo "creating $NIX_LINK" >&2
    set _NIX_DEF_LINK /nix/var/nix/profiles/default
    /nix/store/4yn9bxlscdvbzjmdkhfnxd9a3jm8phns-coreutils-8.23/bin/ln -s "$_NIX_DEF_LINK" "$NIX_LINK"
  end

  set -U fish_user_paths $NIX_LINK/bin $NIX_LINK/sbin $fish_user_paths
    
  # Subscribe the user to the Nixpkgs channel by default.
  if test ! -e $HOME/.nix-channels
    echo "https://nixos.org/channels/nixpkgs-unstable nixpkgs" > $HOME/.nix-channels
  end

  # Append ~/.nix-defexpr/channels/nixpkgs to $NIX_PATH so that
  # <nixpkgs> paths work when the user has fetched the Nixpkgs
  # channel.
  set -x NIX_PATH $NIX_PATH nixpkgs=$HOME/.nix-defexpr/channels/nixpkgs

  # Set $SSL_CERT_FILE so that Nixpkgs applications like curl work.
  if test -e /etc/ssl/certs/ca-bundle.crt
    set -x SSL_CERT_FILE /etc/ssl/certs/ca-bundle.crt
  else if test -e /etc/ssl/certs/ca-certificates.crt
    set -x SSL_CERT_FILE /etc/ssl/certs/ca-certificates.crt
  else if test -e $NIX_LINK/etc/ca-bundle.crt
    set -x SSL_CERT_FILE $NIX_LINK/etc/ca-bundle.crt
  end
end