workflows/labels: fix merge conflict label

The previous implementation had two problems:
- When switching from /search to /pulls, we disabled the additional GET
on each single pull request - which causes no test merge commit creation
for all PRs. This means, merge conflicts will not actually be detected.
- By using `item` in the pull-request triggered case, this goes back to
`context.payload.pull_request`, which is the state *at the beginning* of
the workflow run. But this renders our "let's wait 3 minutes before
checking merge_commit_sha" logic void. While we wait for 3 minutes, we
still use the *old* value afterwards...

Just making the extra request every time simplifies the logic and solves
both problems.

(cherry picked from commit 59ac9479e4)

.github/workflows/labels.yml

@@ -136,12 +136,9 @@ jobs:
                 const pull_number = item.number
                 const issue_number = item.number
 
-                // The search result is of a format that works for both issues and pull requests and thus
-                // does not have all fields of a full pull_request response. Notably, it is missing `head.sha`,
-                // which we need to fetch the workflow run below. This field is already available non-search sources.
-                // This API request is also important for the merge-conflict label, because it triggers the
+                // This API request is important for the merge-conflict label, because it triggers the
                 // creation of a new test merge commit. This is needed to actually determine the state of a PR.
-                const pull_request = item.head ? item : (await github.rest.pulls.get({
+                const pull_request = (await github.rest.pulls.get({
                   ...context.repo,
                   pull_number
                 })).data