Use systemd-sysusers to create user & group
    
    This duplicates the sysusers config from upstream but only installs the
    upstream one. The configs should be kept in sync.
    
    We can not use the upstream one as it requires pre-processing that will
    only happen at a later time in the build process.
    
    See: https://docs.fedoraproject.org/en-US/packaging-guidelines/UsersAndGroups/#_dynamic_allocation
    
    https://src.fedoraproject.org/rpms/flatpak/pull-request/8