nix-prefetch fails on nur-packages. error: Could not find Nixpkgs path: /tmp/nur-packages/pkgs/top-level

[milahu]

nix-prefetch fails on nur-packages

error: Could not find Nixpkgs path: /tmp/nur-packages/pkgs/top-level

to reproduce

cd /tmp
git clone https://github.com/milahu/nur-packages
cd nur-packages
git checkout 670a6637e9488bbd2f8bc00e3c8dc58a9fe1f1bc
./pkgs/tools/archivers/sevenzip/update.sh

update.sh fails at

NEW_FREE_HASH=$(nix-prefetch -f "$NIXPKGS_ROOT" -E "$attr_path.src" --url "$URL")

which evals to

nix-prefetch -f /tmp/nur-packages -E sevenzip.src --url https://7-zip.org/a/7z2200-src.tar.xz

ideally, nix-prefetch should not rely on the internal structure of nixpkgs (pkgs/top-level)
and only use default.nix or flake.nix as entry point

just like nix-build

nix-build /tmp/nur-packages -A sevenzip

workaround:

- nix-prefetch -f "$NIXPKGS_ROOT" -E "$attr_path.src" --url "$URL"
+ nix-prefetch -f "<nixpkgs>" -E "(import $NIXPKGS_ROOT {}).$attr_path.src" --url "$URL"