Fix the careful job with a workaround

ref: RalfJung/cargo-careful#31

ci/careful.sh

@@ -3,6 +3,9 @@ set -euxo pipefail
 IFS=$'\n\t'
 cd "$(dirname "$0")"/..
 
-export RUSTFLAGS="${RUSTFLAGS:-} -Z randomize-layout"
+# TODO: Use the system's default linker instead of rust-lld, which recently
+# became the default until this cargo-careful bug is fixed:
+#   https://github.com/RalfJung/cargo-careful/issues/31
+export RUSTFLAGS="${RUSTFLAGS:-} -Z randomize-layout -Z linker-features=-lld"
 
 cargo careful test --all --all-features --exclude benchmarks -- --test-threads=1