fix: NixOS module warning (#459)

flake.nix

@@ -303,7 +303,7 @@ EOF
 
             package = mkOption {
               type = types.package;
-              default = self.packages.${pkgs.system}.default;
+              default = self.packages.${pkgs.stdenv.hostPlatform.system}.default;
               description = "The Gitea Mirror package to use";
             };