[CI]Fix nightly clang installation following previous attempt (#5907)

### What this PR does / why we need it?
This PR fixes the issue where the previous PR
https://github.com/vllm-project/vllm-ascend/pull/5733 failed to install
Clang in nightly environment.

- vLLM version: v0.13.0
- vLLM main:
bde38c11df

---------

Signed-off-by: Meihan-chen <jcccx.cmh@gmail.com>

.github/workflows/_e2e_nightly_single_node.yaml

@@ -126,6 +126,9 @@ jobs:
       - name: Install Ascend toolkit & triton_ascend
         shell: bash -l {0}
         run: |
+          apt-get update && apt-get -y install clang-15
+          update-alternatives --install /usr/bin/clang clang /usr/bin/clang-15 20
+          update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-15 20
           BISHENG_NAME="Ascend-BiSheng-toolkit_aarch64_20260105.run"
           BISHENG_URL="https://vllm-ascend.obs.cn-north-4.myhuaweicloud.com/vllm-ascend/${BISHENG_NAME}"
           wget -O "${BISHENG_NAME}" "${BISHENG_URL}" && chmod a+x "${BISHENG_NAME}" && "./${BISHENG_NAME}" --install && rm "${BISHENG_NAME}"

.github/workflows/_e2e_nightly_single_node_models.yaml

@@ -107,7 +107,7 @@ jobs:
       - name: Install Ascend toolkit & triton_ascend
         shell: bash -l {0}
         run: |
-          apt-get -y install clang-15
+          apt-get update && apt-get -y install clang-15
           update-alternatives --install /usr/bin/clang clang /usr/bin/clang-15 20
           update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-15 20          
           BISHENG_NAME="Ascend-BiSheng-toolkit_aarch64_20260105.run"

tests/e2e/nightly/multi_node/scripts/run.sh

@@ -127,7 +127,7 @@ install_extra_components() {
 
 install_triton_ascend() {
     echo "====> Installing triton_ascend"
-    apt-get install -y clang-15
+    apt-get update && apt-get install -y clang-15
     update-alternatives --install /usr/bin/clang clang /usr/bin/clang-15 20
     update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-15 20
     clang -v