Evaluation can be slow, which is why flakes-based tools usually cache the evaluation results. (There aren’t going to be duplicate downloads in any case.)

There’s no shebang support in nix3 (the new, flakes-native CLI with a single “nix” command). The closest you can get is probably “nix run”—can’t be distilled down to a single file, but can fetch and cache even online Git repos (use “nix registry pin” to freeze the commit hash).