[Bugfix][CI] Fix aisbench installation to avoid Gitee authentication (#7536)

### What this PR does / why we need it?
- Pass GITEE_USERNAME (var) and GITEE_TOKEN (secret) as Docker build
  args in nightly image build so Dockerfile can authenticate to Gitee
- In Dockerfile.nightly.a2/a3, embed credentials into clone URL to
  avoid auth failure during `git clone`
- In single-node and multi-node PR test workflows, backup the
  pre-installed benchmark from the nightly image before wiping
  vllm-ascend, then restore it instead of re-cloning from Gitee,
  which is inaccessible from fork PR contexts

### Does this PR introduce _any_ user-facing change?

### How was this patch tested?

- vLLM version: v0.18.0
- vLLM main:
8b6325758c

Signed-off-by: hfadzxy <starmoon_zhang@163.com>

.github/workflows/_e2e_nightly_single_node.yaml

@@ -88,6 +88,7 @@ jobs:
         if: ${{ inputs.is_pr_test }}
         run: |
           pip uninstall -y vllm vllm-ascend || true
+          cp -r /vllm-workspace/vllm-ascend/benchmark /tmp/aisbench-backup || true
           rm -rf /vllm-workspace/vllm /vllm-workspace/vllm-ascend
 
       - name: Checkout vllm-project/vllm repo
@@ -132,9 +133,9 @@ jobs:
 
       - name: Install aisbench
         if: ${{ inputs.is_pr_test }}
-        shell: bash -l {0} 
+        shell: bash -l {0}
         run: |
-          git clone -b v3.0-20250930-master --depth 1 https://gitee.com/aisbench/benchmark.git /vllm-workspace/vllm-ascend/benchmark
+          cp -r /tmp/aisbench-backup /vllm-workspace/vllm-ascend/benchmark
           cd /vllm-workspace/vllm-ascend/benchmark
           pip install pytest asyncio pytest-asyncio
           pip install -e . -r requirements/api.txt -r requirements/extra.txt