ci: include pre-release label for RCs #1334

Job	Run time
setup	12s
validate-example (actor)	4m 13s
validate-example (configuration)	1m 18s
validate-example (conversation)	1m 28s
validate-example (crypto)	1m 8s
validate-example (dist-scheduler)	59s
validate-example (grpc-service)	2m 8s
validate-example (hello-world)	52s
validate-example (pubsub)	1m 51s
validate-example (streamsub)	58s
validate-example (service)	1m 53s
validate-example (socket)	46s
validate-example (workflow)	1m 30s
validate-example (workflow-parallel)	58s
	20m 14s

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