CB-7311 Fix xcode project manipulation on Windows host

When adding plugins on Windows, plugman will insert incorrect paths,
which also contain backslashes into the xcode project files. This breaks
builds of the iOS app in Xcode and requires manual editing of the
project file.

On all non-Windows platforms, where the path separator is already `/`,
the performed replace should be a noop, on Windows it fixes the paths in
the project file.

github: close #109