Hardcode /usr/sbin/alternatives for Flatpak builds
    
        /usr/sbin/alternatives is not under the Flatpak build's %{_prefix},
        which is set to /app.
    
        (See <https://pagure.io/packaging-committee/issue/848> "Clarify the
        use of path macros with respect to build dependencies").
    
        Found when trying to do a LibreOffice Flatpak build from RHEL RPM
        specs, which includes java-1.8.0-openjdk among its components.