workflows/labels: label rebuilds immediately

This fixes labeling in the context of the `pull_request` trigger. Of
course, this was supposed to immediately label rebuilds after eval
finishes, but we somehow lost this along the way. Rebuilds are still
labeled fairly soon, because the scheduled trigger will pick up the same
PR within 10 minutes again and then apply the rebuild labels. But of
course, immediate is better.

The reason this happened is, that we're looking at `item.pull_request`
only. This is the correct distinction between "issue items" and "pull
request items", which we both get back from the /issues endpoint. But
the payload for the `pull_request*` event doesn't contain this, so
labeling in this case was treated like an issue and skipped the whole
pull request part.

ci/labels/labels.cjs

@@ -235,7 +235,7 @@ module.exports = async function ({ github, context, core, dry }) {
 
       const itemLabels = {}
 
-      if (item.pull_request) {
+      if (item.pull_request || context.payload.pull_request) {
         stats.prs++
         Object.assign(itemLabels, await handlePullRequest(item))
       } else {