Add CI checks with LLVM sysroot #2029

Job	Run time
build (--test-successful-rustc --nb-parts 2 --current-part 0)	51s
build (--mini-tests)	51s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	51s
build (--std-tests)	47s
build (--test-libcore)	53s
success_gcc12	3s
	4m 16s

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