Fix data race on expanded postings Cache #10489

Job	Run time
CodeQL	6m 48s
test	15m 47s
build	4m 0s
lint	4m 16s
integration-configs-db	2m 48s
integration (requires_docker)	7m 15s
integration (integration_alertmanager)	4m 12s
integration (integration_backward_compatibility)	10m 18s
integration (integration_memberlist)	7m 42s
integration (integration_querier)	15m 30s
integration (integration_ruler)	5m 44s
integration (integration_query_fuzz)	4m 54s
deploy_website	0s
deploy	0s
	1h 29m 14s

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