enforce no-merge policy via triagebot

src/tools/miri/.github/workflows/ci.yml

@@ -208,7 +208,7 @@ jobs:
           git push -u origin $BRANCH
       - name: Create Pull Request
         run: |
-          PR=$(gh pr create -B master --title 'Automatic sync from rustc' --body '')
+          PR=$(gh pr create -B master --title 'Automatic sync from rustc' --body '' --label subtree-sync)
           ~/.local/bin/zulip-send --user $ZULIP_BOT_EMAIL --api-key $ZULIP_API_TOKEN --site https://rust-lang.zulipchat.com \
             --stream miri --subject "Cron Job Failure (miri, $(date -u +%Y-%m))" \
             --message "A PR doing a rustc-pull [has been automatically created]($PR) for your convenience."

src/tools/miri/triagebot.toml

@@ -9,3 +9,6 @@ allow-unauthenticated = [
 
 # Gives us the commands 'ready', 'author', 'blocked'
 [shortcut]
+
+[no-merges]
+exclude_labels = ["rollup", "subtree-sync"]