we get the path object storage config like:
```c++
db::config::get_conf_sub("object_storage.yaml").native()
```
so, the default path should be $SCYLLA_CONF/object_storage.yaml.
in this change, it is corrected.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#15406