cupertino_http: Use `sharedDarwinSource` #6

Job	Run time
health / changelog / run changelog health check	31s
health / license / run license health check	38s
health / version / run version health check	21s
health / breaking / run breaking health check	29s
health / coverage / run coverage health check	33s
health / do-not-submit / run do-not-submit health check	36s
health / leaking / run leaking health check	30s
health / comment	8s
	3m 46s

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