ci: Update mergify to not loop

According to Mergify support this should prevent the looping we've seen.

.github/mergify.yml

@@ -27,12 +27,23 @@ pull_request_rules:
       queue:
         name: default
         method: squash
+  - name: Add merge train label
+    conditions:
+      - "queue-position >= 0"
+    actions:
       comment:
-        message: |
-          This PR has been added to the merge queue, and will be merged soon.
+          message: |
+            This PR has been added to the merge queue, and will be merged soon.
       label:
         add:
           - on-merge-train
+  - name: Remove merge train label
+    conditions:
+      - "queue-position = -1"
+    actions:
+      label:
+        remove:
+          - on-merge-train
   - name: Handle security critical PRs
     conditions:
       - "label=SR-Risk"
@@ -96,9 +107,6 @@ pull_request_rules:
       comment:
         message: |
           Merge failed. Please see automated check logs for more details.
-      label:
-        remove:
-          - on-merge-train
   - name: Nag changesets
     conditions:
       - and: