Fix backport pipeline (#5610)

.backportrc.json

@@ -7,8 +7,8 @@
   },
   "autoMerge": true,
   "autoMergeMethod": "squash",
-  "prTitle": "[{targetBranch}] Cherry pick: {commitMessages}",
-  "prDescription": "Backports the following commits to `{targetBranch}`:\n{commitMessages}",
+  "prTitle": "[{{targetBranch}}] Cherry pick: {{commitMessages}}",
+  "prDescription": "Backports the following commits to `{{targetBranch}}`:\n{{commitMessages}}",
   "sourcePRLabels": ["backported"],
   "targetPRLabels": ["$1-backport"],
   "autoAssign": false,