build.cfg hard-set GPU_DRIVER="" , and build.sh sources build.cfg after the
environment is already set, so `env GPU_DRIVER=nvidia-590 ./build.sh` was
silently clobbered to "" (no NVIDIA branch baked). There is no --gpu-driver
flag in build.sh, so the environment is the only way to set it.
Use GPU_DRIVER="${GPU_DRIVER:-}" so an env value is honored; default stays "".
Verified: env nvidia-590 -> nvidia-590; no env -> "". Replaces the
detached-worktree workaround applied during the live build.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>