Config: Cache dir is inconsistant on Unix and Windows

[charlesbaynham]

dvc version = 0.93.0
Platform = Ubuntu and Windows 10

On Windows, calling dvc cache dir ..\xyz results in an entry in .dvc/config like

[cache]
    dir = ..\..\xyz

On Linux it results in

[cache]
    dir = ../../xyz

This inconsistency results in noise in the git history when you work sometimes on Windows, sometimes on Linux.

Manually editing the file to a Posix-style path on Windows works fine.