chore: robustify PR release workflow (#3051)

the workflow is triggered not only by pull-request-CI-runs but also by
others. These should be skipped.

Also, no need to query the Github API to get the pull request number and
head sha, they are part of the payload, it seems.

.github/workflows/pr-release.yml

@@ -16,20 +16,16 @@ on:
 jobs:
   on-success:
     runs-on: ubuntu-latest
-    if: github.event.workflow_run.conclusion == 'success' && github.repository == 'leanprover/lean4'
+    if: github.event.workflow_run.conclusion == 'success' && github.event.workflow_run.event == 'pull_request' && github.repository == 'leanprover/lean4'
     steps:
-      - name: Retrieve PR number and head commit
+      - name: Set PR number and head commit
         uses: actions/github-script@v7
         id: workflow-info
         with:
           script: |
-            const reply = await github.rest.actions.getWorkflowRun({
-                owner: context.repo.owner,
-                repo: context.repo.repo,
-                run_id: context.payload.workflow_run.id,
-            })
-            core.setOutput('pullRequestNumber', reply.data.pull_requests[0].number)
-            core.setOutput('sourceHeadSha',     reply.data.pull_requests[0].head.sha)
+            // console.log(`context.payload: ${JSON.stringify(context.payload, null, 2)}`)
+            core.setOutput('pullRequestNumber', context.payload.workflow_run.pull_requests[0].number)
+            core.setOutput('sourceHeadSha',     context.payload.workflow_run.pull_requests[0].head.sha)
 
       - name: Download artifact from the previous workflow.
         if: ${{ steps.workflow-info.outputs.pullRequestNumber != '' }}