Fix potential race condition in the EPUB navigator #1180

Job	Run time
Navigator UI Tests	14m 52s
Lint	2m 2s
Integration (Local)	2m 50s
Build	8m 15s
Integration (Swift Package Manager)	5m 48s
Integration (Carthage)	0s
Integration (Local)	3m 50s
Navigator UI Tests	14m 0s
Integration (Carthage)	-1104s
Build	8m 15s
Integration (Swift Package Manager)	5m 48s
Lint	2m 2s
	49m 18s

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