[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/dockerfiles/Dockerfile.nightly.a2

@@ -20,6 +20,8 @@ FROM quay.io/ascend/vllm-ascend:main
 ARG PIP_INDEX_URL="https://mirrors.tuna.tsinghua.edu.cn/pypi/web/simple"
 ARG AIS_BENCH_TAG="v3.0-20250930-master"
 ARG AIS_BENCH_URL="https://gitee.com/aisbench/benchmark.git"
+ARG GITEE_USERNAME=""
+ARG GITEE_TOKEN=""
 
 # Define environments
 ENV DEBIAN_FRONTEND=noninteractive
@@ -35,7 +37,8 @@ RUN export PIP_EXTRA_INDEX_URL=https://mirrors.huaweicloud.com/ascend/repos/pypi
     python3 -m pip cache purge
 
 # Install benchmark tools
-RUN git clone -b ${AIS_BENCH_TAG} --depth 1 ${AIS_BENCH_URL} /vllm-workspace/vllm-ascend/benchmark && \
+RUN CLONE_URL=$(echo "${AIS_BENCH_URL}" | sed "s|https://|https://${GITEE_USERNAME}:${GITEE_TOKEN}@|") && \
+    git clone -b ${AIS_BENCH_TAG} --depth 1 "${CLONE_URL}" /vllm-workspace/vllm-ascend/benchmark && \
     cd /vllm-workspace/vllm-ascend/benchmark && \
     pip install -e . -r requirements/api.txt -r requirements/extra.txt && \
     python3 -m pip cache purge