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.