apply: support magic rollback for remote machines

[water-sucks]

Feature Description

deploy-rs has a "magic rollback" feature for remote deploys that can automatically rollback a configuration in case a subsequent SSH connection fails after a timeout.

nixos-cli already has an auto-rollback feature for profiles in case switch-to-configuration fails, but this is not properly supported for remote hosts in the case that SSH connectivity is severed, either due to network changes or something else.

As such, we can improve on this by implementing our own transparent "magic rollback" feature. Ideally, the remote machine will have a watchdog of some sort that watches for a file to be removed by an SSH connection after switch-to-configuration runs. This will have to be a self-contained binary, or part of the nixos activate routine somehow.

A flag to disable magic rollback should also exist, in case this is not desired behavior.

Help

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Issues

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