Toolbx: Don't leak DISTTAG and FGC into the container
    
    While DISTTAG and FGC are not very common names for environment
    variables, they are still not properly namespaced, and hence pollute the
    global namespace for environment variables.  The less that happens, the
    better.
    
    https://pagure.io/pungi-fedora/pull-request/1213
    
    Signed-off-by: Debarshi Ray <rishi@fedoraproject.org>