build: Inherit flags from rustc #2072

Job	Run time
Clippy	7s
Test CUDA support	1m 38s
Test wasm (wasm32-unknown-unknown)	13s
Test build-std (aarch64-tvos)	2m 14s
MSRV (ubuntu-latest)	22s
Test build-std (aarch64-tvos-sim)	2m 22s
MSRV (windows-latest)	47s
Rustfmt	7s
semver-checks	9s
Test build-std (x86_64-tvos)	2m 10s
MSRV (macos-latest)	32s
Test wasm32-wasip1-thread	30s
Test (stable)	1m 38s
Test (beta)	1m 26s
Test (nightly)	1m 15s
Test (linux32)	2m 17s
Test (macos)	5m 37s
Test (aarch64-macos)	1m 57s
Test (aarch64-ios)	1m 34s
Test (win32)	3m 5s
Test (win64)	4m 12s
Test (mingw32)	3m 47s
Test (mingw64)	3m 11s
Test (windows-2019)	4m 19s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	3m 37s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	5m 32s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	3m 0s
	57m 38s

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