The +nightly syntax is a rustup proxy feature, so a distro-packaged cargo won’t even parse it. Nightly is a different compiler every day.

export RUSTC_BOOTSTRAP=1 and you wouldn't need rustup or nightly. It will work with your stable distro package.

clap = "4" and anyhow = "1" are ranges, and there’s no Cargo.lock.

you can do clap = "=4.x.x", ditto for all deps, direct and indirect.

Nightly is a different compiler every day.

Nothing about that script is fixed except its text.

Is nix finished free-standing software?