CUDA 12.8 deprecates architectures below 75. Presumably a future
version will remove it. Prepare infrastructure to avoid relying on
hard-coded arch 50 in this test.
This extends commit 63a5460faf (ci: Tell CudaOnly.Architecture test what
specific architecture to use, 2025-04-21).
With CUDAARCHS there's the possibility of tests being run with multiple
architectures or having CMAKE_CUDA_ARCHITECTURES set with NVCC despite CMP0104
being OLD.
Fix a few tests to work properly in such cases.
Robert Maynard