(treesit-query-validate): Fix reusing the output buffer

* lisp/treesit.el (treesit-query-validate): Fix the "Buffer is
read-only" error when an output buffer already exists.

lisp/treesit.el

@@ -2380,7 +2380,8 @@ to the offending pattern and highlight the pattern."
          (with-current-buffer buf
            (let* ((data (cdr err))
                   (message (nth 0 data))
-                  (start (nth 1 data)))
+                  (start (nth 1 data))
+                  (inhibit-read-only t))
              (erase-buffer)
              (insert (treesit-query-expand query))
              (goto-char start)