Checkout the PR when triggering try from PR comments (#30069)

Fixes #30067.

.github/workflows/linux-wpt.yml

@@ -25,8 +25,16 @@ jobs:
         chunk_id: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]
     steps:
       - uses: actions/checkout@v3
+        if: github.event_name != 'issue_comment'
         with:
           fetch-depth: 2
+      # This is necessary to checkout the pull request if this run was triggered
+      # via an `issue_comment` action on a pull request.
+      - uses: actions/checkout@v3
+        if: github.event_name == 'issue_comment'
+        with:
+          ref: refs/pull/${{ github.event.issue.number }}/head
+          fetch-depth: 2
       - uses: actions/download-artifact@v3
         with:
           name: release-binary