Mark pipeline build step with the downstream build result

[stuartrowe]

When wait: true is set, add a WarningAction to the build step FlowNode when the downstream build isn't successful even when propagate: false is also set.

Also, log the build result when the downstream run completes.

Implements https://issues.jenkins.io/browse/JENKINS-70623

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[stuartrowe]

@jglick @dwnusbaum would either of you be able to review this or suggest someone else? I'm not sure who's maintaining this currently.

[stuartrowe]

Would it be possible to get a review soon please?