add apps to support the nix run command (#46)

flake.nix

@@ -7,10 +7,7 @@
   outputs = {self, nixpkgs, flake-utils}: flake-utils.lib.eachDefaultSystem (system:
     let
       pkgs = import nixpkgs { inherit system; };
-    in
-  {
-    packages.default = pkgs.stdenv.mkDerivation
-    {
+      yaMusic = pkgs.stdenv.mkDerivation {
         name = "yandex-download-music";
         version = "v1.5";
         src = ./.;
@@ -51,6 +48,10 @@
             --prefix PERL5LIB : $PERL5LIB
         '';
       };
-  });
-
+    in
+    {
+      packages.default = yaMusic;
+      apps.default = flake-utils.lib.mkApp { drv = yaMusic; };
+    }
+  );
 }