ci(workflows): specify pull_request_target branches (#41741)

Author: caugner

.github/workflows/pr-rebase-needed.yml

@@ -3,6 +3,8 @@ name: "PR conflicts"
 on:
   push:
   pull_request_target:
+    branches:
+      - main
     types: [synchronize]
 
 permissions:

.github/workflows/system-file-changes.yml

@@ -2,6 +2,8 @@ name: System file changes
 
 on:
   pull_request_target:
+    branches:
+      - main
     paths:
       - ".github/workflows/**"
       - ".github/CODEOWNERS"