$ nix registry list | rg -i nixpkgs
system flake:nixpkgs path:/nix/store/c39q7pww80997lz0fyqxpvsr29q6f7ld-source
global flake:nixpkgs github:NixOS/nixpkgs/nixpkgs-unstable
That seems to match more or less what I'm seeing too then, interesting.
Seems the pack phase isn't using the correct directory, build phase output is Utils -> /build/afbw66jz03aj2fa6cllz2aa4qrf9ay69-source/src/Utils/bin/Release/net8.0/linux-x64/Utils.dll but then the pack phase is erroring with /build/afbw66jz03aj2fa6cllz2aa4qrf9ay69-source/src/Utils/bin/Release/net8.0/Utils.dll' to be packed was not found on disk.