workflows/labels: fix on older PRs

When we made the switch from eval.yml to pr.yml we adjusted the labels
job as well - but didn't take into account that we also need to deal
with old PRs at the same time.

Here, we fallback to another API request to get a run for eval.yml when
we can't find one for pr.yml.

(cherry picked from commit d00d3190d6)

.github/workflows/labels.yml

@@ -152,7 +152,17 @@ jobs:
                     status: prEventCondition ? 'in_progress' : 'success',
                     exclude_pull_requests: true,
                     head_sha: pull_request.head.sha
-                  })).data.workflow_runs[0]?.id
+                  })).data.workflow_runs[0]?.id ??
+                    // TODO: Remove this after 2025-09-17, at which point all eval.yml artifacts will have expired.
+                    (await github.rest.actions.listWorkflowRuns({
+                      ...context.repo,
+                      // In older PRs, we need eval.yml instead of pr.yml.
+                      workflow_id: 'eval.yml',
+                      event: 'pull_request_target',
+                      status: 'success',
+                      exclude_pull_requests: true,
+                      head_sha: pull_request.head.sha
+                    })).data.workflow_runs[0]?.id
 
                   // Newer PRs might not have run Eval to completion, yet. We can skip them, because this
                   // job will be run as part of that Eval run anyway.