Motivation for this change

This is an attempt to fix some issues that got mentioned in #79975 with the new fetchCargoTarball fetcher.

The primary issue seems to be that validateCargoDeps is checked during postUnpack when cargoPatches is not applied yet and builds that have to modify Cargo.lock will therefore fail the check.

Things done

I tested the cargo, rls and clippy builds which use the cargoVendorDir flag, rink and git-dit (update wip) which use cargoPatches to create the Cargo.lock and other crates like cargo-web or parinfer-rust which use the upstream Cargo.lock.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.