Stop using xtask for clippy (#13223)

This fixes an extra 10 second delay when needing to recompile xtask, and
allows passing arbitrary clippy args (like --allow-dirty)

Release Notes:

- N/A

.zed/tasks.json

@@ -1,7 +1,7 @@
 [
   {
     "label": "clippy",
-    "command": "cargo",
-    "args": ["xtask", "clippy"]
+    "command": "./script/clippy",
+    "args": []
   }
 ]