Unsuitable default value to `bundle_root` in `ckpt_export`

[KumoLiu]

Introduced by #6965 which fixed the bundle_root to os.getcwd() without updating.

[2023-10-12T08:17:01.366Z] Traceback (most recent call last):
[2023-10-12T08:17:01.366Z]   File "/root/.local/share/virtualenvs/MONAI-zoo-premerge-CHR6GQKS/lib/python3.8/site-packages/monai/bundle/config_item.py", line 377, in evaluate
[2023-10-12T08:17:01.366Z]     return eval(value[len(self.prefix) :], globals_, locals)
[2023-10-12T08:17:01.366Z]   File "<string>", line 1, in <module>
[2023-10-12T08:17:01.366Z]   File "/root/.local/share/virtualenvs/MONAI-zoo-premerge-CHR6GQKS/lib/python3.8/site-packages/torch/serialization.py", line 771, in load
[2023-10-12T08:17:01.366Z]     with _open_file_like(f, 'rb') as opened_file:
[2023-10-12T08:17:01.366Z]   File "/root/.local/share/virtualenvs/MONAI-zoo-premerge-CHR6GQKS/lib/python3.8/site-packages/torch/serialization.py", line 270, in _open_file_like
[2023-10-12T08:17:01.366Z]     return _open_file(name_or_buffer, mode)
[2023-10-12T08:17:01.366Z]   File "/root/.local/share/virtualenvs/MONAI-zoo-premerge-CHR6GQKS/lib/python3.8/site-packages/torch/serialization.py", line 251, in __init__
[2023-10-12T08:17:01.366Z]     super(_open_file, self).__init__(open(name, mode))
[2023-10-12T08:17:01.366Z] FileNotFoundError: [Errno 2] No such file or directory: './models/search_code_18590.pt'