ci: include pre-release label for RCs #1347

Job	Run time
setup	12s
validate-example (actor)	2m 6s
validate-example (configuration)	1m 34s
validate-example (conversation)	1m 27s
validate-example (crypto)	1m 20s
validate-example (dist-scheduler)	1m 4s
validate-example (grpc-service)	2m 14s
validate-example (hello-world)	1m 1s
validate-example (pubsub)	1m 54s
validate-example (streamsub)	56s
validate-example (service)	1m 53s
validate-example (socket)	59s
validate-example (workflow)	1m 27s
validate-example (workflow-parallel)	55s
	19m 2s

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