Use external lego binary rather than built-in Go package

[stgraber]

Lego with DNS-01 pulls a lot of dependencies which results in a pretty significant increase in size to the binary and ultimately our base memory footprint.

To fix that, the easiest is to interact with the external lego binary.

[accuser]

Quite happy to take this on - it would be straightforward to change the implementation to use the lego binary. But I assume that this will require a change to the incusd package too?

[stgraber]

@accuser once we land the change in Incus, I'll update the Zabbly packaging to include a build of lego.

[stgraber]

@accuser assigned it to you, thanks!

[adamcstephens]

Probably a good idea to make this new binary requirement explicit in the release notes.

[stgraber]

Yeah, I'll try to remember to put a notes for packagers section.