Use RUSTC_WRAPPER if no other wrapper is provided #957

Job	Run time
Test CUDA support	22s
MSRV (ubuntu-latest)	2m 35s
MSRV (windows-latest)	3m 22s
Test aarch64-apple-tvos	1m 28s
Clippy	21s
Rustfmt	11s
Test (stable)	21s
Test (beta)	18s
Test (nightly)	12s
Test (linux32)	27s
Test (macos)	24s
Test (aarch64-ios)	20s
Test (win32)	19s
Test (win64)	19s
Test (mingw32)	14s
Test (mingw64)	19s
Test (windows-2019)	14s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	22s
	12m 8s

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