Support *-unknown-linux-gnu under `hermetic_cc_toolchain` #1

Job	Run time
Test wasm (wasm32-unknown-unknown)	16s
Clippy	11s
Test CUDA support	25s
Test build-std (aarch64-tvos)	1m 59s
Test build-std (aarch64-tvos-sim)	1m 49s
MSRV (ubuntu-latest)	2m 41s
Test build-std (x86_64-tvos)	1m 55s
Rustfmt	11s
semver-checks	2m 23s
MSRV (windows-latest)	3m 16s
MSRV (macos-latest)	2m 51s
Test (stable)	1m 19s
Test (beta)	1m 27s
Test (nightly)	1m 21s
Test (linux32)	1m 32s
Test (macos)	2m 44s
Test (aarch64-macos)	1m 36s
Test (aarch64-ios)	1m 16s
Test (win32)	3m 19s
Test (win64)	3m 1s
Test (mingw32)	3m 1s
Test (mingw64)	3m 3s
Test (windows-2019)	3m 16s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	2m 52s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	3m 12s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	3m 9s
	54m 5s

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