add nix-prefetch-git to app PATH

flake.nix

@@ -37,11 +37,15 @@
             horizon-gen-nix = hfinal.callCabal2nix "horizon-gen-nix" ./. { };
           };
       };
+      horizon-gen-nix-bin = pkgs.writers.writeBashBin "horizon-gen-nix" ''
+        export PATH=${pkgs.nix-prefetch-git}:$PATH
+        ${hsPkgs.horizon-gen-nix}/bin/horizon-gen-nix
+      '';
     in
     {
       apps.default = {
         type = "app";
-        program = "${hsPkgs.horizon-gen-nix}/bin/horizon-gen-nix";
+        program = "${horizon-gen-nix-bin}/bin/horizon-gen-nix";
       };
       devShells.default = hsPkgs.horizon-gen-nix.env.overrideAttrs (attrs: {
         buildInputs = attrs.buildInputs ++ [