Only check CLA on PRs to master.

.github/workflows/cla.yml

@@ -4,6 +4,8 @@ on:
     types: [created]
   pull_request_target:
     types: [opened,closed,synchronize]
+    branches:
+      - master
 
 jobs:
   CLAssistant:
@@ -33,4 +35,4 @@ jobs:
           #custom-pr-sign-comment: 'The signature to be committed in order to sign the CLA'
           #custom-allsigned-prcomment: 'pull request comment when all contributors has signed, defaults to **CLA Assistant Lite bot** All Contributors have signed the CLA.'
           #lock-pullrequest-aftermerge: false - if you don't want this bot to automatically lock the pull request after merging (default - true)
-          #use-dco-flag: true - If you are using DCO instead of CLA
+          #use-dco-flag: true - If you are using DCO instead of CLA