rPackages.gifski: fixed build

      cargoDeps = pkgs.rustPlatform.fetchCargoTarball {

        src = attrs.src;

        sourceRoot = "gifski/src/myrustlib";

        hash = "sha256-vBrTQ+5JZA8554Aasbqw7mbaOfJNQjrOpG00IXAcamI=";

        hash = "sha256-e6nuiQU22GiO2I+bu0muyICGrdkCLSZUDHDz2mM2hz0=";

      };

      cargoRoot = "src/myrustlib";