Reduce steady-state allocations in ui_stack_system #38377

Job	Run time
miri	7m 38s
ci	2m 21s
build (windows-latest)	4m 35s
toml	2m 16s
build (ubuntu-latest)	2m 1s
check-bevy-internal-imports	5s
build (macos-latest)	3m 12s
check-cfg	1m 40s
check-doc	7m 59s
check-missing-examples-in-docs	44s
run-examples-macos-metal	4m 25s
typos	5s
check-missing-features-in-docs	49s
markdownlint	1m 9s
check-compiles	2m 31s
msrv	1m 26s
build-wasm	48s
	43m 44s

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