Run nixos-channel-scripts on a CI

[domenkozar]

The main benefit here would be more transparency into the channel update process so that anyone can see if it failed and what went wrong by looking at the logs.

We briefly discussed with @grahamc and @edolstra and it's only a matter of figuring out how to pull it off.

[lukegb]

Either using something like buildkite for doing this, or even "just" having a Loki instance that we ingest the logs into so we can display them in Grafana would be good.