People often ask how you can pass a value from one build step to another in a Flow, or generally to retrieve information from builds. If you are willing to write the value to an archived file in the first build, you can use a function like

def artifact(build, name) {
  new File(build.artifactsDir, name).text.trim();
}

This works, but the Flow plugin should define something like this (or better) out of the box.

(Also https://github.com/jenkinsci/jenkins/pull/736 would change the Jenkins API here, so it is better that the plugin take care of that detail.)