allow setting `nvcc` program via `CUDACXX` #1256

Job	Run time
Clippy	19s
MSRV (ubuntu-latest)	21s
MSRV (windows-latest)	45s
Test CUDA support	33s
Test build-std (aarch64-tvos)	2m 26s
Rustfmt	10s
Test build-std (aarch64-tvos-sim)	3m 11s
Test build-std (x86_64-tvos)	2m 45s
Test (stable)	27s
Test (beta)	28s
Test (nightly)	42s
Test (linux32)	1m 22s
Test (macos)	1m 26s
Test (aarch64-macos)	45s
Test (aarch64-ios)	57s
Test (win32)	1m 22s
Test (win64)	2m 0s
Test (mingw32)	1m 31s
Test (mingw64)	1m 25s
Test (windows-2019)	2m 24s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	1m 36s
	26m 55s

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