code: synchronized @CheckForNull RunExecution peek() {
          Executor e = Executor.currentExecutor();
          if (e != null) {
              Stack<RunExecution> s = stack.get(e);
              if (s != null && !s.isEmpty()) {
                  return s.peek();
              }
          }
          return null;
      }
docs: Looks up the currently running build, if known.
  <p>While most {@link Run} implementations do add a {@link RunExecution} to the stack for the duration of the build,
  those which have a different implementation of {@link Run#run} (or which do additional work after {@link Run#execute} completes)
  may not consistently or ever keep an execution on the stack.
  In such cases this method will return null, meaning that {@link CheckPoint#block(BuildListener, String)} and {@link CheckPoint#report} will do nothing.
  @return a running build, or null if one has not been recorded