Fix: musl build failures (UPX install permission denied)

Build-only fix: the release pipeline's musl binary build jobs were failing when trying to install UPX. No user-facing output changes.

🐛 Bug fixes

musl builds failed with permission denied installing UPX: The "Install UPX (Linux)" step called apt-get without sudo. Non-musl Linux builds (linux-x64, linux-arm64) run inside a .NET SDK Noble Docker container as root, so the bare apt-get call worked. musl builds (linux-musl-x64, linux-musl-arm64) run directly on the GitHub-hosted runner as a non-root user, so the call failed with permission denied. The fix adds a uid check: when running as root (uid 0), apt-get is called directly; otherwise sudo apt-get is used. Simply always using sudo is not viable because the .NET SDK Noble container used by non-musl builds has no sudo binary.