Add missing `await` to `sleep` calls (#18763) #1472

Job	Run time
Build and push image for linux/arm64	2m 45s
Build and push image for linux/amd64	2m 40s
Push merged images to docker.io/matrixdotorg/synapse	25s
Push merged images to ghcr.io/element-hq/synapse	34s
	6m 24s

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