Attempting to execute the GitHubCommitStatusSetter outside of a node block results in the following error:
 

org.jenkinsci.plugins.workflow.steps.MissingContextVariableException: Required context class hudson.FilePath is missing
Perhaps you forgot to surround the code with a step that provides this, such as: node

This is unfortunate, because all the step does is send an HTTP request to GitHub, which can be done from the Jenkins master, and thus doesn't require a node. In fact, a cursory reading of the code suggests that the request is made from the master. So the step should not require a node in order to run.