disable OsPath build flag on windows for now

disable OsPath build flag on windows for now

Test suite failure looks like this:

        fatal: could not open '\\?\C:\Users\appveyor\AppData\Local\Temp\1\hash-cc81b41d-dfda-4ae8-904b-b531742443cc' for reading: No such file or directory
        fatal: could not open '\\?\C:\Users\appveyor\AppData\Local\Temp\1\hash-cc81b41d-dfda-4ae8-904b-b531742443cc' for reading: No such file or directory
        fatal: could not open '\\?\C:\Users\appveyor\AppData\Local\Temp\1\hash-cc81b41d-dfda-4ae8-904b-b531742443cc' for reading: No such file or directory
        fatal: could not open '\\?\C:\Users\appveyor\AppData\Local\Temp\1\hash-cc81b41d-dfda-4ae8-904b-b531742443cc' for reading: No such file or directory
        fatal: could not open '\\?\C:\Users\appveyor\AppData\Local\Temp\1\hash-cc81b41d-dfda-4ae8-904b-b531742443cc' for reading: No such file or directory
        fatal: could not open '\\?\C:\Users\appveyor\AppData\Local\Temp\1\hash-cc81b41d-dfda-4ae8-904b-b531742443cc' for reading: No such file or directory
        fatal: could not open '\\?\C:\Users\appveyor\AppData\Local\Temp\1\hash-cc81b41d-dfda-4ae8-904b-b531742443cc' for reading: No such file or directory
        fatal: could not open '\\?\C:\Users\appveyor\AppData\Local\Temp\1\hash-cc81b41d-dfda-4ae8-904b-b531742443cc' for reading: No such file or directory
        fatal: could not open '\\?\C:\Users\appveyor\AppData\Local\Temp\1\hash-cc81b41d-dfda-4ae8-904b-b531742443cc' for reading: No such file or directory
        fatal: could not open '\\?\C:\Users\appveyor\AppData\Local\Temp\1\hash-cc81b41d-dfda-4ae8-904b-b531742443cc' for reading: No such file or directory
        fatal: could not open '\\?\C:\Users\appveyor\AppData\Local\Temp\1\hash-cc81b41d-dfda-4ae8-904b-b531742443cc' for reading: No such file or directory
        git-annex: fd:4: Data.ByteString.hGetLine: end of file
        git-annex: user error (git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","hash-object","-w","--no-filters","--stdin-paths"] exited 128)

This is apparently in Git.HashObject, and probably in hashBlob, which uses a
temp file with a name starting with "hash", but I have not been able to tell
what's wrong.

I don't understand where the "\\?" path prefix (windows UNC-style path)
is coming from in the path that gets fed into git hash-file, or why git
hash-file can't open the file.