[Build] Reduce nvcc_threads to 1 for linux cuda plugin ci

[Tianlei Wu (tianleiwu)]

Linux cuda plugin CI failed sometime due to OOM. This reduce nvcc_threads to 1 to avoid the issue.

Example failed jobs:
https://github.com/microsoft/onnxruntime/actions/runs/31229218069/job/93029489524?pr=31704

It seems that runner has no log after the following (looks like OOM):

[1133/1762] Building CUDA object CMakeFiles/onnxruntime_providers_cuda_plugin_sm90_tma.dir/onnxruntime_src/onnxruntime/contrib_ops/cuda/llm/fpA_intB_gemm/launchers/fpA_intB_gemm_launcher_7.generated.cu.o
[1134/1762] Building CUDA object CMakeFiles/onnxruntime_providers_cuda_plugin_sm90_tma.dir/onnxruntime_src/onnxruntime/contrib_ops/cuda/llm/fpA_intB_gemm/launchers/fpA_intB_gemm_launcher_8.generated.cu.o
[1135/1762] Building CUDA object CMakeFiles/onnxruntime_providers_cuda_plugin_sm90_tma.dir/onnxruntime_src/onnxruntime/contrib_ops/cuda/llm/fpA_intB_gemm/launchers/fpA_intB_gemm_launcher_9.generated.cu.o