xcbuild: exit with error if --find fails

pkgs/development/tools/xcbuild/wrapper.nix

···
          --toolchain | -toolchain) shift ;;
          --find | -find | -f)
            shift
-           command -v $1 ;;
+           command -v $1 || exit 1 ;;
          --log | -log) ;; # noop
          --verbose | -verbose) ;; # noop
          --no-cache | -no-cache) ;; # noop