Avoid using _prefix macro for /usr/lib/debug dir
    
    In case of flatpak module builds, _prefix is redefined as /app.
    rpm-build however hardcodes /usr/lib/debug without using the _prefix
    macro, so do the same here to match this.
    
    This fixes building Python as a flatpak module.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1737933#c14