Use RUSTC_WRAPPER if no other wrapper is provided (#918) #971

Job	Run time
MSRV (ubuntu-latest)	23s
Test CUDA support	26s
MSRV (windows-latest)	42s
Clippy	16s
Test aarch64-apple-tvos	2m 17s
Rustfmt	8s
Test (stable)	20s
Test (beta)	26s
Test (nightly)	25s
Test (linux32)	44s
Test (macos)	37s
Test (aarch64-ios)	1m 11s
Test (win32)	1m 13s
Test (win64)	1m 29s
Test (mingw32)	1m 21s
Test (mingw64)	1m 26s
Test (windows-2019)	1m 51s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	1m 52s
	17m 7s

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