Make sure the artifacts are re-created (#674)

diff --git a/tool/grind.dart b/tool/grind.dart
index 209ecad..d2d8bf4 100644
--- a/tool/grind.dart
+++ b/tool/grind.dart
@@ -241,7 +241,8 @@
 
   // Copy both to the project directory.
   final artifactsDir = getDir('artifacts');
-  await artifactsDir.create();
+  delete(artifactsDir);
+  artifactsDir.createSync();
 
   final sdkJsPath = path.join(flutterSdkPath,
       'bin/cache/flutter_web_sdk/flutter_web_sdk/kernel/amd-canvaskit-html/dart_sdk.js');