Add Windows CI workflow

[gabriel-samfira]

The tests on Windows for a number of packages seem to fail. A lot of them are a simple fix of just doing a filepath.FromSlash() on expected paths, but some are legitimately failing. Looking at the tested code that fails. Looks like most of it is in regards to following symlinks or walking paths.

As this code is being used in buildkit in https://github.com/moby/buildkit/blob/master/cache/util/fsutil.go#L99, this will have to be fixed before we can safely add Windows support to builkit.

Looking into it now, and will post back an update as soon as I have something.