UI Materials: don't reserve items on phase from inside the loop #31957

Job	Run time
ci	2m 41s
miri	7m 48s
build (windows-latest)	4m 35s
build (ubuntu-latest)	2m 4s
build (macos-latest)	4m 53s
run-examples-on-windows-dx12	14m 8s
check-bevy-internal-imports	4s
check-doc	5m 28s
check-missing-examples-in-docs	30s
check-missing-features-in-docs	40s
markdownlint	1m 24s
check-compiles	2m 38s
msrv	54s
build-wasm	41s
	48m 28s

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