Arnar Guðmundsson contributed to the hmemcpy/nixpkgs repository by enhancing the build process for the the-way package. He reworked the package to source its code from GitHub rather than crates.io, enabling full access to unit tests and improving test coverage. Arnar implemented platform-aware build checks using Nix, allowing tests and verification steps to run conditionally based on the host environment. His work focused on build systems and package management, ensuring that the package could be reliably tested across platforms. The changes were well-documented and addressed the need for more robust, cross-platform testability within the Nix ecosystem.