When running
```
set UV_CACHE_DIR=%LOCALAPPDATA%\uv\cache-foo && uv venv venv
```
in windows CMD, the error would be just
```
error: The system cannot find the path specified. (os error 3)
```
The problem is that the first action in the cache dir is adding the tag,
and the `cachedir` crate is using `std::fs` instead of `fs_err`. I've
copied the two functions we use from the crate and changed the import
from `std::fs` to `fs_err`.
The new error is
```
error: failed to open file `C:\Users\Konstantin\AppData\Local\uv\cache-foo \CACHEDIR.TAG`
Caused by: The system cannot find the path specified. (os error 3)
```
which correctly explains the problem.
Closes#3280