Add additional cargo options cargoHome and enableSccache

[sakompella]

Description

• Create cargoHome option to set CARGO_HOME environment variable
  • Update the settings option to place the resulting config.toml file into the arbitrary
  • Note: due to my own knowledge I naively use home.file here, but theoretically people could set CARGO_HOME to any directory, although probably not common. I am not sure how to reflect this.
• Create enableSccache to set cfg.settings.build.rustc_wrapper = "${pkgs.sccache}/bin/sccache" (using sccache is very common, as it's a very easy speedup for rust build times

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- cargo

  • note: it seem unrelated firefox tests are failing with nix run .#tests -- test-all, since 7eca7f7 (the latest master commit).

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.