Remove `build_sysroot` folder #16

Job	Run time
build_system	24s
duplicates	5s
build (gcc-13.deb, --mini-tests)	6m 10s
build (gcc-13.deb, --std-tests)	6m 16s
build (gcc-13.deb, --test-libcore)	13m 31s
build (gcc-13.deb, --extended-rand-tests)	9m 14s
build (gcc-13.deb, --extended-regex-example-tests)	6m 25s
build (gcc-13.deb, --extended-regex-tests)	12m 20s
build (gcc-13.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	16m 39s
build (gcc-13.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	13m 36s
build (gcc-13.deb, --projects)	16m 45s
build (gcc-13-without-int128.deb, --mini-tests)	5m 40s
build (gcc-13-without-int128.deb, --std-tests)	6m 3s
build (gcc-13-without-int128.deb, --test-libcore)	13m 47s
build (gcc-13-without-int128.deb, --extended-rand-tests)	8m 58s
build (gcc-13-without-int128.deb, --extended-regex-example-tests)	6m 38s
build (gcc-13-without-int128.deb, --extended-regex-tests)	12m 0s
build (gcc-13-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	11m 25s
build (gcc-13-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	11m 52s
build (gcc-13-without-int128.deb, --projects)	16m 18s
	3h 14m 6s

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