Motivation for this change

Dependencies specified as git repos are not working due to missing output in .cargo/config. This patch saves the output in fetchcargo and includes parts of it in the one regenerated in buildRustPackage. Note that the temporary cargo-config file must be removed prior to executing cargo update.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

I've setup a repository to demonstrate the working configuration: https://github.com/jbboehr/nix-fetch-cargo-update

I believe this implementation is a bit brittle. A "perfect" implementation would probably use a toml parser to remove or replace the relevant sections in .cargo/config.

I tried running nix-shell -p nox --run "nox-review wip" but it crashed, maybe it affects too many packages?

Fixes #41518