rclone: set meta.mainProgram

pkgs/applications/networking/sync/rclone/default.nix

···
     homepage = "https://rclone.org";
     changelog = "https://github.com/rclone/rclone/blob/v${version}/docs/content/changelog.md";
     license = licenses.mit;
+    mainProgram = "rclone";
     maintainers = with maintainers; [ marsam SuperSandro2000 ];
   };
 }