ci: skip approval for nightly gb200 runs, keep for manual triggers (#22768)

.github/workflows/nightly-72-gpu-gb200.yml

@@ -1,8 +1,8 @@
 name: Nightly Test (GB200 72GPU)
 
-# NOTE: This workflow is intentionally cron-only.
-# It must NOT be triggered manually (no workflow_dispatch) to prevent
-# individuals from queuing arbitrary jobs on the shared GB200 cluster.
+# NOTE: Nightly (schedule) runs require no approval.
+# Manual (workflow_dispatch) runs are gated by the gb200-ci environment
+# to prevent individuals from queuing arbitrary jobs on the shared GB200 cluster.
 on:
   schedule:
     - cron: '0 2 * * *'  # 2 AM UTC daily (offset from other nightly runs)
@@ -53,7 +53,7 @@ jobs:
   prepare-image:
     needs: setup
     if: github.repository == 'sgl-project/sglang'
-    environment: gb200-ci
+    environment: ${{ github.event_name == 'workflow_dispatch' && 'gb200-ci' || '' }}
     runs-on: 72-gpu-gb200
     outputs:
       squash_file: ${{ steps.import.outputs.squash_file }}