Andreas Hatziiliou focused on improving build reliability for the mlkem-c-aarch64 repository by addressing a critical issue where changes to environment variables failed to trigger necessary recompilations. He implemented an Environment Variable-Aware Rebuild Trigger within the Makefile, ensuring that environment variable values are tracked, defaults are applied, and state is restored after including cached configuration. This approach reduced the risk of stale binaries and improved the feedback loop in continuous integration. Andreas demonstrated depth in build systems and Makefile orchestration, applying environment variable handling to enable more reproducible, reliable builds and streamline incremental build processes for the project.