The documentation in help.jelly links to the Javadoc for hudson.model.AbstractBuild for the build variable, but it is actually a `hudson.model.Run`.

Documentation:
https://github.com/jenkinsci/groovy-postbuild-plugin/blob/master/src/main/resources/org/jvnet/hudson/plugins/groovypostbuild/GroovyPostbuildRecorder/help.jelly#L8

Declaration:
https://github.com/jenkinsci/groovy-postbuild-plugin/blob/master/src/main/java/org/jvnet/hudson/plugins/groovypostbuild/GroovyPostbuildRecorder.java#L110