Only add WarningAction when propagate: true

[stuartrowe]

Fix for https://issues.jenkins.io/browse/JENKINS-70844 which is a knock on from #107.

Looking at the implementation of BuildConditions in declarative pipelines, any WarningAction attached to a node within a stage will trigger the Unsuccessful condition. See BuildCondition.java#L165.

In order to maintain the previous behavior, only add the WarningAction when propagte: true.

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[stuartrowe]

@car-roll @olamy please review when you have a chance. Thanks!

[stuartrowe]

Alternative fix which avoids undoing the behavior added by #107: jenkinsci/pipeline-model-definition-plugin#598.

[stuartrowe]

CI job timed out / aborted on the linux-11 branch. Closing and re-opening to trigger another build.

[stuartrowe]

Triggering another PR build.