util: simplify `ReusableBoxFuture::try_set` logic #11098

Job	Run time
Check README	3s
clippy	20s
docs (windows-latest)	20s
docs (ubuntu-latest, --cfg tokio_taskdump, --cfg tokio_taskdump)	19s
fmt	19s
minrust	20s
basic checks	0s
build tokio for redox-os	0s
asan	0s
check-external-types (${{ matrix.os }})	0s
check-fuzzing	0s
check-spelling	0s
check unexpected lints and cfgs	0s
check tokio full --internal-mt-counters	0s
cross-check	0s
cross-check-tier3	0s
cross-test-without-parking_lot	0s
cross-test-with-parking_lot	0s
features ${{ matrix.name }}	0s
build loom tests	0s
minimal-versions	0s
miri	0s
Check tokio --feature-powerset --depth 2 on i686-unknown-linux-gnu without AtomicU64	0s
Test tokio --all-features on i686-unknown-linux-gnu without AtomicU64	0s
semver	0s
Test hyper	0s
Run integration tests for each feature	0s
compile tests with parking lot send guards	0s
Test Quinn	0s
test tokio full	0s
test tokio full --unstable	0s
test tokio full --unstable --taskdump	0s
test all crates in the workspace with all features	0s
test all crates in the workspace with all features and panic=abort	0s
valgrind	0s
test tokio for wasm32-unknown-unknown	0s
${{ matrix.target }}	0s
build tokio for x86_64-fortanix-unknown-sgx	0s
all systems go	0s
	1m 41s

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