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GH-45514: [CI][C++][Docs] Set CUDAToolkit_ROOT explicitly in debian-docs (#45520)

### Rationale for this change

CMake's `FindCUDAToolkit.cmake` uses `/usr/lib/cuda/` as the default prefix but Debian's `nvidia-cuda-dev` uses `/usr` as prefix.

### What changes are included in this PR?

Set `CUDAToolkit_ROOT=/usr` explicitly.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #45514

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>