cupertino_http: Use `sharedDarwinSource` #4

Job	Run time
health / breaking / run breaking health check	25s
health / changelog / run changelog health check	37s
health / license / run license health check	37s
health / version / run version health check	22s
health / coverage / run coverage health check	38s
health / do-not-submit / run do-not-submit health check	37s
health / leaking / run leaking health check	27s
health / comment	5s
	3m 48s

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