LocalStore fsspec adding extra path stuff in Windows

```
from obstore.fsspec import FsspecStore
fs = FsspecStore('file')
fs.exists('c:/temp/xx.txt')
```

-->

```
Generic LocalFileSystem error: Unable to walk dir: IO error for operation on C:\C:\temp\xx.txt: The filename, directory name, or volume label syntax is incorrect. (os error 123)

Debug source:
Generic {
store: "LocalFileSystem",
source: UnableToWalkDir {
source: Error {
depth: 0,
inner: Io {
path: Some(
"C:\\C:\\temp\\xx.txt",
),
err: Os {
code: 123,
kind: InvalidFilename,
message: "The filename, directory name, or volume label syntax is incorrect.",
},
},
},
},
}
```

More info in #555

Note: I am happy to try to fix this and maybe some other fsspec bugs myself. (I'm not yet aware of what those might be, but once this is fixed I'll be able to find out more as I try to get obstore working for my use case.) However, I can't find any contribution guidelines. LMK if there's some kind of readme on that topic, or what info I need to get going as a contributor if you're open to external help. Thanks.

Reproduce the issue through obstore.fsspec.FsspecStore with fs.exists('c:/temp/xx.txt') on Windows, then inspect how the local filesystem path is handled. Compare the behavior with the related discussion in #555. Done means the drive prefix is not duplicated and the existence check works for this Windows path.

Written by the indexing model from the issue text.