"System" attribute of installed files is not preserved

When installing a file that has the system attribute on a Windows PC, the installed file does not get the attribute. The issue seems to occur while extracting the file, as the packaged archive still has the attribute set.

It looks like this could be fixed by setting the attributes of installed files at the end of ExtractCallback::GetStream() in lib7z_facade.cpp after they've been written, using the Windows API, https://docs.microsoft.com/de-de/windows/win32/api/fileapi/nf-fileapi-setfileattributesa. If not all attributes should be copied the unwanted ones can be masked out.

(This is currently causing issues for me while trying to deploy a cygwin installation, as those use the system attribute to mark symlinks, which are used e.g. for /bin/awk and the list of CA certificates...)