serving: SinkBinding doesn't work with KServices that uses BYO revision names

Describe the bug

When using a SinkBinding with a Knative Service as subject that uses BYO revision names (i.e. setting the revision name from the client), then no K_SINK environment variable is injected to the KService.

Expected behavior

K_SINK should be injected to the Pod Template as a container env var regardless whether a Knative Service is configured for BYO revision names or not.

To Reproduce

kn service create random --image rhuss/random:1.0
kn broker create default
kn source binding create my-sb --subject Service:serving.knative.dev/v1:random --sink broker:default
kn service describe random --verbose | grep K_SINK

This will get no result as kn used BYO revision names by default. If switching over to server side generated names, then a K_SINK env var is injected.

# Switch over to server side generated names
kn service update random --revision-name ""
kn service describe random --verbose | grep K_SINK

        Env:    K_SINK=http://broker-ingress.knative-eventing.svc.cluster.local/default/default

Knative release version

Eventing 0.17.3 (running on minikube)