Bug in xjoin with Windows pathnames
Currently, `xjoin` function has a bug with local Windows pathnames: instead of returning the OS-dependent join pathname, it always returns it in POSIX format.

```python
from datasets.download.streaming_download_manager import xjoin

path = xjoin("C:\\Users\\USERNAME", "filename.txt")
```

Join path should be:
```python
"C:\\Users\\USERNAME\\filename.txt"
```
However it is:
```python
"C:/Users/USERNAME/filename.txt"
```