Use the compiler returned by build_sysroot for benchmarking #1056

Job	Run time
abi_cafe (macos-latest, aarch64-apple-darwin)	3m 31s
abi_cafe (macos-latest, x86_64-apple-darwin)	10m 22s
abi_cafe (ubuntu-latest, x86_64-unknown-linux-gnu)	3m 9s
abi_cafe (windows-latest, x86_64-pc-windows-msvc)	5m 35s
	22m 37s

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