[0.26] Use sender image promoted by OpenShift CI (knative#910)

Align the default sender image to one promoted by OpenShift CI.

This aligns defaults with openshift-knative/serverless-operator#1337

Further refs.:

 - openshift/release#24476
 - openshift/release#24549

hack/build.sh.d/kn-plugin-event.sh

@@ -3,8 +3,8 @@
 knEventVersion="$(grep 'knative.dev/kn-plugin-event ' "$(basedir)/go.mod" \
   | head -n 1 \
   | sed -s 's/.* \(v.[\.0-9]*\).*/\1/')"
-readonly upstream_sender_image='gcr.io/knative-releases/kn-event-sender@sha256:8d562077864d6490438ac5b35dd352c6ffa6cd089e81fa06aaf6fe56a0374196'
-readonly sender_image="${KN_PLUGIN_EVENT_SENDER_IMAGE:-${upstream_sender_image}}"
+readonly default_sender_image='registry.ci.openshift.org/knative/release-0.26:client-plugin-event-sender'
+readonly sender_image="${KN_PLUGIN_EVENT_SENDER_IMAGE:-${default_sender_image}}"
 export EXTERNAL_LD_FLAGS="${EXTERNAL_LD_FLAGS:-} \
 -X knative.dev/kn-plugin-event/pkg/metadata.Image=${sender_image} \
--X knative.dev/kn-plugin-event/pkg/metadata.Version=${knEventVersion}"
+-X knative.dev/kn-plugin-event/pkg/metadata.Version=${knEventVersion}"