coreutils: don't use bare sed on darwin

it will be BSD sed instead of GNU sed

pkgs/tools/misc/coreutils/default.nix

@@ -19,7 +19,7 @@ let
     };
 
     # The test tends to fail on btrfs and maybe other unusual filesystems.
-    postPatch = ''
+    postPatch = stdenv.lib.optionalString (!stdenv.isDarwin) ''
       sed '2i echo Skipping dd sparse test && exit 0' -i ./tests/dd/sparse.sh
     '';