Remove `build_sysroot` folder #28

Job	Run time
build (--mini-tests)	3m 27s
build (--std-tests)	3m 24s
build (--test-libcore)	10m 12s
build (--test-successful-rustc --nb-parts 2 --current-part 0)	7m 59s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	7m 52s
	32m 54s

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