Make sure the dst dir doesn't exist when calling shutil.copytree (#29952)

In #29603, we replaced distutils.dir_util.copy_tree with shutil.copytree since the former is deprecated. There is a big difference between these two methods that we ignored and caused test failures: distutils.dir_util.copy_tree works regardless whether dst dir exists or not but shutil.copytree expects that the dst dir doesn't exist.

Signed-off-by: Jiajun Yao <jeromeyjj@gmail.com>