ghprbCommentBody contains the wrong comment when polling github

Our build is setup to poll github every 2 minutes for new comments on pull requests. A trigger phrase is setup to trigger a build when a comment is seen with the trigger phrase. Recently, we moved to Jenkins pipelines and have started parsing the `ghprbCommentBody` variable to allow PR users to control what gets built. However, we found that if more comments are made after the one containing the trigger phrase, then the `ghprbCommentBody` variable will contain the text of the last comment on the PR rather than the text of the comment that triggered the build.

We're currently running with ghprb v1.36.2 -- the latest version at the time of writing this is v1.39.0 but looking at the changelog it does not appear that this problem was reported/fixed.