[workflows] Only run code formatter on the main branch (llvm#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.

(cherry picked from commit 2193c95)

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

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