ci: include pre-release label for RCs #1335

Job	Run time
setup	13s
validate-example (actor)	2m 9s
validate-example (configuration)	1m 15s
validate-example (conversation)	1m 26s
validate-example (crypto)	1m 8s
validate-example (dist-scheduler)	56s
validate-example (grpc-service)	2m 10s
validate-example (hello-world)	44s
validate-example (pubsub)	1m 53s
validate-example (streamsub)	56s
validate-example (service)	1m 54s
validate-example (socket)	43s
validate-example (workflow)	1m 36s
validate-example (workflow-parallel)	1m 3s
	18m 6s

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