fix: use backslash for path on windows

[wenym1]

In PR tokio-rs/tokio#6558, I was trying to use rustversion.

But it fails the CI on windows-latest due to the following error.

couldn't read \\?\D:\a\tokio\tokio\target\debug\build\rustversion-2d44b26e828f2c8d\out/version.expr: The filename, directory name, or volume label syntax is incorrect. (os error 123)

This is because currently the version is loaded from a path with fixed / as path separator, but on windows the path separator should be \.

In this PR, we will change to use \ as path separator for windows.

In a separate CI run that uses the commit patched from this PR, the test passes.

[dtolnay]

Thanks for the PR!

As is, this change would break cross-compilation from non-Windows to Windows because target_os = "windows" would be true in that case but \ paths do not make sense on the non-Windows host. I put up #51 with an implementation that decides the path separator based on the host platform, rather than the target platform.