Install signal-desktop in macOS via nixpkgs

[kachick]

Resolves GH-843

[kachick]

Ah, it has unfree license only in the darwin package...

https://github.com/NixOS/nixpkgs/pull/348165/files#diff-05921dc46b537c59c8a76dfc3c3e9a3a1fd93345ee5bff8573aae36dedf719bcR49

        (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: Package ‘signal-desktop-7.29.0’ in /nix/store/riqkpszjqk02bi1wppfg8ip5xvh102qd-source/pkgs/applications/networking/instant-messengers/signal-desktop/signal-desktop-darwin.nix:35 has an unfree license (‘agpl3Only free unfree’), refusing to evaluate.

       a) To temporarily allow unfree packages, you can use an environment variable
          for a single invocation of the nix tools.

            $ export NIXPKGS_ALLOW_UNFREE=1

          Note: When using `nix shell`, `nix build`, `nix develop`, etc with a flake,
                then pass `--impure` in order to allow use of environment variables.

       b) For `nixos-rebuild` you can set
         { nixpkgs.config.allowUnfree = true; }
       in configuration.nix to override this.

       Alternatively you can configure a predicate to allow specific packages:
         { nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
             "signal-desktop"
           ];
         }

       c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
         { allowUnfree = true; }
       to ~/.config/nixpkgs/config.nix.

[kachick]

https://github.com/AsamK/signal-cli