blacksmith.sh: Migrate workflows to Blacksmith

[blacksmith-sh]

This PR migrates some selected workflows to Blacksmith.

@jescalan note - this should be a pretty low effort way to make our CI faster & less expensive, with a very easy revert if it's not working out for us for any reason

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Latest commit: 58783dd

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[jescalan]

This is gonna need a logic change within our variables in order to actually work here, just a note

[nikosdouvlis]

Need to tweak a few GH variables, blocking so we don't break CICD accidentally :)