Increase root filesystem size to 3GB for Atomic variant
    
    Fixes: #6765
    
    The default size of filesystem provided by lorax is 2GB and the
    installtree size is gone up to 2.7GB which is failing to copy files
    from installtree to created root filesystem.
    
    NOTE: Just increased for x86_64 and ppc64le since aarch64 is fine.
    
    Signed-off-by: Mohan Boddu <mboddu@redhat.com>