fix: restore missing `run_as_child` #815

Job	Run time
get-leptos-changed / Detect Source Change	7s
CI (integrations/actix) / Run ci (nightly)	16m 3s
CI (integrations/utils) / Run ci (nightly)	15m 3s
CI (integrations/viz) / Run ci (nightly)	15m 29s
CI (integrations/axum) / Run ci (nightly)	16m 28s
CI (leptos) / Run ci (nightly)	1h 9m 23s
CI (leptos_config) / Run ci (nightly)	14m 56s
CI (leptos_dom) / Run ci (nightly)	33m 7s
CI (leptos_macro) / Run ci (nightly)	25m 57s
CI (leptos_hot_reload) / Run ci (nightly)	19m 12s
CI (leptos_reactive) / Run ci (nightly)	40m 42s
CI (leptos_server) / Run ci (nightly)	26m 13s
CI (meta) / Run ci (nightly)	16m 10s
CI (router) / Run ci (nightly)	18m 21s
CI (server_fn) / Run ci (nightly)	20m 53s
CI (server_fn_macro) / Run ci (nightly)	14m 18s
CI (server_fn/server_fn_macro_default) / Run ci (nightly)	14m 35s
	6h 16m 57s

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