[workflows] Only run code formatter on the main branch (#80348)

Modifying a cherry-picked patch to fix code formatting issues can be
risky, so we don't typically do this. Therefore, it's not necessary to
run this job on the release branches.

Author: tstellar

.github/workflows/pr-code-format.yml

@@ -1,5 +1,8 @@
 name: "Check code formatting"
 on: pull_request_target
+  branches:
+    - main
+
 permissions:
   pull-requests: write