### 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>
886756aea0