Fix Sync impl of FuturesUnordered #2334

Job	Run time
cargo +1.56 build	2m 24s
codegen	41s
cargo bench	2m 32s
cargo +1.36 build (futures-{core, io, sink})	3m 50s
cargo +stable build	1m 1s
cargo +beta build	1m 27s
cargo hack check --feature-powerset	4m 32s
cargo minimal-versions build	40s
cargo +nightly build	1m 18s
cargo build --target thumbv7m-none-eabi	45s
cargo test (ubuntu-latest)	4m 38s
cargo clippy	36s
cargo build --target thumbv6m-none-eabi	46s
cargo test (macos-latest)	7m 11s
cargo test (windows-latest)	6m 56s
cargo doc	28s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	5m 2s
cargo fmt	8s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	8m 47s
cargo miri test	13m 33s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	9m 3s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	5m 29s
cargo test -Z sanitizer=address	4m 12s
cargo test -Z sanitizer=memory	3m 8s
cargo test -Z sanitizer=thread	3m 2s
	1h 32m 9s

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