Allow prepare_windows to run off main thread. #35881

Job	Run time
build (windows-latest)	4m 50s
check-bevy-internal-imports	3s
check-doc	5m 30s
build (ubuntu-latest)	1m 45s
check-missing-examples-in-docs	49s
build (macos-latest)	2m 26s
ci	1m 38s
miri	6m 29s
run-examples-on-windows-dx12	14m 13s
toml	3s
check-missing-features-in-docs	31s
markdownlint	1m 10s
check-compiles	2m 21s
msrv	48s
build-wasm	36s
	43m 12s

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