Also drop diffutils from the `inherit (prevStage)` overrides in
pkgs/stdenv/linux/default.nix. Once the package lives in pkgs/by-name,
that pin gives the top-level `diffutils` attribute a source location
even though it is defined via `_internalCallByNamePackageFile`, which
nixpkgs-vet rejects (NPV-102). Dropping the pin lets the by-name
auto-definition stand on its own, at the cost of no longer reusing the
bootstrap-stage build (a stdenv-class rebuild, hence staging).
Co-authored-by: Philip Taron <philip.taron@gmail.com>
Assisted-by: Claude Code (claude-opus-4-8)
