feat(nix): add slack

Signed-off-by: Seongmin Lee <[e-mail]>

nix/flake.nix

···
     system = "aarch64-darwin";
     pkgs = import nixpkgs {
       inherit system;
+      config.allowUnfree = true;
       overlays = [
         (import ./overlays/jpm.nix)
         (import ./overlays/bleve.nix)
···
         pkgs.iina
         pkgs.macmon
         pkgs.docker
+        pkgs.slack
       ] else []);
     };
   };