ci(cuda): build only for CI runner GPU arch and raise parallelism

[MrLi000001]

The CUDA CI built every .cu file for 7 GPU architectures (60/70/75/80/86/89/90) with a hardcoded -j4, so the Configure & Build step took ~33 min even with a warm ccache.

• Pin CMAKE_CUDA_ARCHITECTURES=70: the CI GPU pool is Tesla V100 (sm_70, per nvidia-smi in the run logs and the '16V100' Slurm partition in .ci/slurm/config.ini). This cuts nvcc work by ~7x.
• Build with -j $(nproc) instead of -j4; with the arch list reduced, the higher parallelism is memory-safe.

Expected: Configure & Build ~33 min -> ~10 min on a cache-cold run.

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[Stardust0831]

This is an effective direction: Configure & Build dropped from about 33 minutes to 18m01s in the actual run. Before merging, please confirm that the entire gpu runner pool guarantees sm_70 and update the ~10-minute estimate with the measured result; otherwise, a heterogeneous runner may not be able to run the binary.