Remove `build_sysroot` folder #18

Job	Run time
duplicates	3s
build_system	30s
build (gcc-13.deb, --mini-tests)	5m 49s
build (gcc-13.deb, --std-tests)	5m 59s
build (gcc-13.deb, --test-libcore)	13m 21s
build (gcc-13.deb, --extended-rand-tests)	8m 24s
build (gcc-13.deb, --extended-regex-example-tests)	6m 48s
build (gcc-13.deb, --extended-regex-tests)	12m 12s
build (gcc-13.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	11m 4s
build (gcc-13.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	11m 19s
build (gcc-13.deb, --projects)	16m 41s
build (gcc-13-without-int128.deb, --mini-tests)	6m 2s
build (gcc-13-without-int128.deb, --std-tests)	5m 52s
build (gcc-13-without-int128.deb, --test-libcore)	14m 18s
build (gcc-13-without-int128.deb, --extended-rand-tests)	8m 35s
build (gcc-13-without-int128.deb, --extended-regex-example-tests)	6m 29s
build (gcc-13-without-int128.deb, --extended-regex-tests)	12m 17s
build (gcc-13-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	11m 29s
build (gcc-13-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	11m 35s
build (gcc-13-without-int128.deb, --projects)	16m 47s
	3h 5m 34s

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