use PR_REVISION for ref

instead of DEFAULT_BRANCH

Author: chmouel

.tekton/trigger-e2e-test.yaml

@@ -62,7 +62,7 @@ spec:
                 }
 
                 payload = {
-                    "ref": DEFAULT_BRANCH,
+                    "ref": PR_REVISION,
                     "inputs": {
                         "target_ref": PR_REVISION
                     }