prevent libgccjit.so download on unsupported os/arch #2084

Job	Run time
duplicates	3s
build (gcc-13.deb, --mini-tests)	5m 21s
build_system	24s
build (gcc-13.deb, --std-tests)	5m 53s
build (gcc-13.deb, --test-libcore)	13m 18s
build (gcc-13.deb, --extended-rand-tests)	8m 30s
build (gcc-13.deb, --extended-regex-example-tests)	6m 10s
build (gcc-13.deb, --extended-regex-tests)	11m 56s
build (gcc-13.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	12m 14s
build (gcc-13.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	12m 0s
build (gcc-13.deb, --projects)	16m 10s
build (gcc-13-without-int128.deb, --mini-tests)	5m 46s
build (gcc-13-without-int128.deb, --std-tests)	5m 41s
build (gcc-13-without-int128.deb, --test-libcore)	13m 45s
build (gcc-13-without-int128.deb, --extended-rand-tests)	9m 15s
build (gcc-13-without-int128.deb, --extended-regex-example-tests)	6m 23s
build (gcc-13-without-int128.deb, --extended-regex-tests)	11m 52s
build (gcc-13-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	12m 18s
build (gcc-13-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	12m 18s
build (gcc-13-without-int128.deb, --projects)	16m 40s
	3h 5m 57s

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