container-toolbox: Use 'toolbx' as VARIANT_ID in os-release(5)
    
    This is meant to distinguish OCI containers and images that are designed
    specifically for Toolbx from others.  Toolbx containers are long-lasting
    pet containers for interactive command line use, which makes them
    substantially different from short-lived containers running services.
    
    Therefore, it can be useful to be able to identify Toolbx containers and
    images when generating statistics about Fedora usage.
    
    https://pagure.io/Fedora-Council/tickets/issue/449
    https://pagure.io/fedora-kickstarts/pull-request/1015