maintainers/scripts/update.nix: derivation is the final station

It does not make sense to look for derivations within derivations,
not even when `recurseForDerivations` is true. Nix does not do that either:

https://github.com/NixOS/nix/blob/ebc024df2287085d48ed6194aa756fd70c07f76c/src/libexpr/get-drvs.cc#L346-L355