Don’t do that. If you’re not setting a hash, nix will fetch this file every time the download cache expires, by default every 2 hours. This kills all the guarantees nix normally gives you. Use pkgs.fetchFromGitHub if you must manually manage your inputs, since it refuses to work without a hash.
Alternatively, use npins - it literally uses the same mechanism for importing an external project, but gives you a reasonable way to update and prevents rookie mistakes like the one you just made.