Avoid over allocating strings.Builder when creating cache key for exp… #10805

Job	Run time
lint	4m 14s
build	4m 1s
CodeQL	6m 48s
test	15m 49s
test-no-race	7m 29s
integration-configs-db	2m 39s
integration (requires_docker)	7m 18s
integration (integration_alertmanager)	4m 15s
integration (integration_backward_compatibility)	10m 59s
integration (integration_memberlist)	7m 27s
integration (integration_querier)	15m 10s
integration (integration_ruler)	5m 48s
integration (integration_query_fuzz)	5m 8s
deploy_website	53s
deploy	3m 15s
	1h 41m 13s

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