Add support for parsing `*FLAGS` with `shlex` #1793

Job	Run time
Test wasm (wasm32-unknown-unknown)	28s
Clippy	23s
Test CUDA support	44s
MSRV (ubuntu-latest)	31s
Test (stable)	1m 14s
MSRV (windows-latest)	1m 6s
semver-checks	18s
Test (beta)	1m 25s
MSRV (macos-latest)	1m 2s
Test (nightly)	1m 14s
Test (linux32)	1m 27s
Rustfmt	13s
Test (macos)	1m 29s
Test build-std (aarch64-tvos)	1m 56s
Test (aarch64-macos)	1m 12s
Test build-std (aarch64-tvos-sim)	1m 54s
Test (aarch64-ios)	1m 12s
Test build-std (x86_64-tvos)	1m 42s
Test (win32)	1m 9s
Test (win64)	1m 9s
Test (mingw32)	1m 13s
Test (mingw64)	1m 16s
Test (windows-2019)	1m 11s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	1m 11s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	1m 14s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	1m 13s
	29m 6s

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