fix(rustfmt): add cargoManifestPath back

[Pyxels]

It seems that #504 removed the application of the option of settings.rust.cargoManifestPath for rustfmt.

This PR readds that option so that setting settings.rust.cargoManifestPath (or the new rustfmt.settings.manifest-path) adds the --manifest-path argument to rustfmt.

hooks = {
  rustfmt.enable = true;
};
settings.rust.cargoManifestPath = "backend/Cargo.toml";
}

now becomes:

"entry": "/nix/store/04q1sh6cia6ar7zap273v3vpisf5mj7q-rustfmt-wrapped/bin/cargo-fmt fmt --all --manifest-path backend/Cargo.toml -- --color always"