ci: Tell CudaOnly.CompileFlags test what specific architecture to use

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).