CI: fix GPU tests (#4311)
- Move to v6.17.0 queue with CUDA 12.4
- Upgrade buildkite plugins
- Replace image-repository option with push
- Use CUDA 11.8 container images, 12.0 for head
- TF nightly has minimum Python version 3.9
Signed-off-by: Enrico Minack github@enrico.minack.dev