[lmi] No security updates yet for debian 'bullseye'

[Greg Chicares]

Vadim--Please see lmi commit e3f989ef703. I'm in a quandary: it seems
wrong not to enable security upgrades; but if I don't, 'apt-get update'
fails, which may be worse (if it therefore fails to retrieve some of
the updates that actually are available for other entries in
/etc/apt/sources.list). Do you know a better way to handle this?