CI: Flowey update to support conditional step evaluation

[benhillis]

There are a couple places where it would be nice to support making various pipeline jobs support conditions. This might be a little interesting because GitHub and ADO handle conditions a little differently.

https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/using-conditions-to-control-job-execution
https://learn.microsoft.com/en-us/azure/devops/pipelines/process/conditions?view=azure-devops

This would be uesful in a handful of places, for example: Updating guide building / publishing to only happen on main branch.

[daprilik]

We have some of this already (see https://openvmm.dev/rustdoc/linux/flowey/node/prelude/struct.NodeCtx.html#method.emit_ado_step_with_condition), but there's certainly more to flesh out here.