Trigger CLA check on push to backport branch.

Author: pikinier20

.github/workflows/cla.yml

@@ -1,10 +1,15 @@
 name: Scala CLA
-on: [pull_request]
+on:
+  pull_request:
+  push:
+    branches:
+      - 'language-reference-backport'
 jobs:
   check:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
     - run: ./project/scripts/check-cla.sh
+      if: github.event_name == 'pull_request'
       env:
         AUTHOR: ${{ github.event.pull_request.user.login }}