CI: Use the pull request number

Author: HaoZeke

.github/workflows/slashdot_trigger.yml

@@ -12,4 +12,4 @@ jobs:
           token: ${{ secrets.ASV_TOK }}
           commands: |
             trigger-asv
-          client-payload: '{"pr_number": "${{ github.event.issue.number }}"}'
+          client-payload: '{"pr_number": "${{ github.event.pull_request.number }}"}'