Run the CLA bot only on comments to PR

Right now, it unnecessarily runs and sends an email when customers open issues.

Not tested, but can test it by opening an issue after this is merged.

Reference:
- https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#issue_comment

.github/workflows/cla.yaml

@@ -7,6 +7,8 @@ on:
 
 jobs:
     CLAAssistant:
+        # This job only runs for pull request comments
+        if: ${{ github.event.issue.pull_request }}
         runs-on: ubuntu-latest
         steps:
             - name: "CLA Assistant"