GH-45514: [CI][C++][Docs] Set CUDAToolkit_ROOT explicitly in debian-docs #49351

Job	Run time
Check minimum supported Arrow C++ Version (15.0.2)	7m 41s
rhub/ubuntu-gcc12:latest	44m 27s
AMD64 Ubuntu 24.04 R 4.4 Force-Tests true	11m 20s
AMD64 Windows C++ RTools 40 ucrt64	25m 15s
AMD64 Windows R release	16m 40s
	1h 45m 23s

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