OFFLINE_WORK is fragile and untested

e.g. on a NanoPi R4S image build, we attempt to install ethtool off the internet. That obviously doesn't work.

While I can fix that one specific issue with ethtool's package, I think fundamentally we would have to have some CI/CD testing of `OFFLINE_WORK`.

Open question: should we remove this flag completely for now, add a warning, or wait until it's fixed?

Start by tracing the OFFLINE_WORK handling in the image-build scripts and the package-install path that attempts to fetch ethtool. Review how CI/CD could exercise offline builds; done means the policy for OFFLINE_WORK is resolved and offline builds no longer attempt internet-dependent installation.

Written by the indexing model from the issue text.