sequoia: fix build by migrating off legacy fetchCargo implementation
sequoia: fix build by migrating off legacy fetchCargo implementation

Currently broken; see #79975 for details. Would also be fixed by #80153
eventually, but since we want to upgrade either way we might as well do so now.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs