osc-test-fbc: add a label "latest" to the on-push pipeline

This was asked to ease identification of the latest build.
Doing it on the on-push pipeline makes sure the "latest" tag
points to a build that was validated and manually created, not
an automated, potentially incomplete build.

Signed-off-by: Julien Ropé <[email protected]>

Author: littlejawa

.tekton/osc-test-fbc-push.yaml

@@ -308,6 +308,8 @@ spec:
         value: $(tasks.build-image-index.results.IMAGE_URL)
       - name: IMAGE_DIGEST
         value: $(tasks.build-image-index.results.IMAGE_DIGEST)
+      - name: ADDITIONAL_TAGS
+        value: ["latest"]
       runAfter:
       - build-image-index
       taskRef: