Rename the workflow step that sets labels to a more appropriate name (#5646)

Currently it's called `assign_reviewers` as it was copied from the
workflow that does said task. But this workflow is setting labels, so
call it `set_labels`.

.github/workflows/auto_label_prs.yaml

@@ -11,7 +11,7 @@ permissions:
   pull-requests: write # For gh to edit labels.
 
 jobs:
-  assign_reviewer:
+  set_labels:
     runs-on: ubuntu-latest
     steps:
       - name: Harden Runner