chore: short-circuit changeset check in merge group context

When running in a merge queue we don't have a single PR, but a group. I
was unable to figure out how to check for changesets on the merge group.

Instead we short-circuit in the merge group context, and rely on the
individual PR check for changesets.

Author: kanej

.github/workflows/check-changeset-added.yml

@@ -26,6 +26,15 @@ jobs:
       - uses: actions/github-script@v7
         with:
           script: |
+            const isMergeGroup = context.eventName === "merge_group";
+
+            // Merge group context
+            if (isMergeGroup) {
+              console.log("Ignore changeset check for merge group.");
+              return;
+            }
+
+            // Single PR context
             const pullNumber = context.issue.number;
 
             const { data: files } = await github.rest.pulls.listFiles({