The current nix expressions perform an Import From Derivation when reading the cargo lockfile due to the interpolation with "{src}", which unfortunately breaks cross-system evaluation (as well as nix flake check).
This PR removes the IFD for Cargo.lock. I hope it doesn't have unintended side effects that I didn't forsee.
Second commit also drops x86_64-darwin from the supported systems set, as it is no longer supported by nixpkgs-unstable (and will be dropped on the next 26.11 release). Both fixes allow nix flake check --all-systems to pass.
