Update changelog

Author: Sean Robertson

CHANGELOG.md

@@ -5,6 +5,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## 4.2.0
+
+* Add: If `buildkite_agent_logs_dir` is unset for the target platform, log to stdout/stderr instead of an arbitrary default file.
+
 ## 4.1.0
 
 * Add: macOS `buildkite_agent_brew_dir` to set where `brew` is installed.