Remove `build_sysroot` folder #20

Job	Run time
build (--mini-tests)	3m 23s
build (--std-tests)	3m 32s
build (--test-libcore)	10m 36s
build (--test-successful-rustc --nb-parts 2 --current-part 0)	7m 32s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	9m 34s
	34m 37s

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