Enable playground tests triggered on cron schedule (#39069)

Author: shunping

.github/workflows/beam_Playground_Precommit.yml

@@ -34,6 +34,7 @@ jobs:
     if: |
       github.event_name == 'workflow_dispatch' ||
       github.event_name == 'pull_request_target' ||
+      (github.event_name == 'schedule' && github.repository == 'apache/beam') ||
       github.event.comment.body == 'Run Playground PreCommit'
     name: ${{ matrix.job_name }} (${{ matrix.job_phrase }})
     runs-on: [self-hosted, ubuntu-24.04, main]