run dependabot workflow only on dependabot branches #2495

Job	Run time
Unit Test (knative/serving)	10m 10s
Unit Test (knative/networking)	5m 1s
Unit Test (knative/eventing)	13m 13s
Unit Test (knative/operator)	7m 37s
Unit Test (knative/client)	7m 41s
Unit Test (knative-extensions/eventing-autoscaler-keda)	5m 11s
Unit Test (knative-extensions/eventing-ceph)	5m 27s
Unit Test (knative-extensions/eventing-github)	6m 15s
Unit Test (knative-extensions/eventing-gitlab)	5m 35s
Unit Test (knative-extensions/eventing-kafka-broker)	9m 54s
Unit Test (knative-extensions/eventing-rabbitmq)	8m 36s
Unit Test (knative-extensions/eventing-redis)	5m 57s
Unit Test (knative-extensions/kn-plugin-admin)	5m 22s
Unit Test (knative-extensions/net-certmanager)	4m 53s
Unit Test (knative-extensions/net-contour)	5m 5s
Unit Test (knative-extensions/net-istio)	6m 9s
Unit Test (knative-extensions/net-kourier)	5m 19s
	1h 57m 25s

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