[CI] Run 4xB200 steps nightly only (#61541)

Signed-off-by: khluu <khluu000@gmail.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

.buildkite/test_areas/disaggregated_mooncake.yaml

@@ -4,6 +4,8 @@ depends_on:
 steps:
 - label: ":nvidia: (B200) Distributed MooncakeConnector PD accuracy"
   key: distributed-mooncakeconnector-pd-accuracy-4-gpus
+  # Nightly only: 4-GPU B200 pods wait hours behind smaller jobs on per-PR runs.
+  optional: true
   timeout_in_minutes: 10
   working_dir: "/vllm-workspace/tests"
   # Runs on b200-k8s because it is the only NVIDIA pool that can carry mooncake
@@ -22,6 +24,7 @@ steps:
   mirror:
     amd:
       label: ":amd: (MI355) Distributed MooncakeConnector HIP PD accuracy"
+      optional: true
       dind: false
       device: mi355_4
       num_devices: 4

.buildkite/test_areas/models_basic.yaml

@@ -161,6 +161,8 @@ steps:
 
 - label: ":nvidia: (4xB200) Kimi K3 Prefix Cache"
   key: kimi-k3-prefix-cache-4xb200
+  # Nightly only: 4-GPU B200 pods wait hours behind smaller jobs on per-PR runs.
+  optional: true
   automatic_shard: true
   timeout_in_minutes: 75
   device: b200-k8s