Fix vulnerability in cross-repo-issue GitHub action (#4324)

Author: zxPhoenix

.github/workflows/cross-repo-issue.yml

@@ -23,9 +23,10 @@ jobs:
           github.event.pull_request.merged
         env:
           GITHUB_TOKEN: ${{ steps.generate_token.outputs.token }}
+          PR_TITLE: ${{ github.event.pull_request.title }}
         run: |
           echo -e "A PR was merged over on PBS-Java\n\n- [https://github.com/prebid/prebid-server-java/pull/${{github.event.number}}](https://github.com/prebid/prebid-server-java/pull/${{github.event.number}})\n- timestamp: ${{ github.event.pull_request.merged_at}}" > msg
           export msg=$(cat msg)
-          gh issue create --repo prebid/prebid-server --title "Port PR from PBS-Java: ${{ github.event.pull_request.title }}" \
+          gh issue create --repo prebid/prebid-server --title "Port PR from PBS-Java: $PR_TITLE" \
               --body "$msg" \
               --label auto