Traditional jobs have a 'builtOn' key as part of the payload however this is noticeably absent in pipeline builds. I feel this is the right behavior as a pipeline can have multiple node{} blocks so a single key would not be very useful. However it seems there is no way to get this information back out other than scraping the test of the console log. It looks like the information is stored in the workflow/<n>.xml files and with a &depth=2 param, some of the information about the individual parts of a workflow run are exposed via REST but the node is absent.