[CI/Build] Make the Dockerfile.cpu file's PIP_EXTRA_INDEX_URL Configurable as a Build Argument (#9252)

Dockerfile.cpu

@@ -26,7 +26,8 @@ RUN pip install intel_extension_for_pytorch==2.4.0
 
 WORKDIR /workspace
 
-ENV PIP_EXTRA_INDEX_URL=https://download.pytorch.org/whl/cpu
+ARG PIP_EXTRA_INDEX_URL="https://download.pytorch.org/whl/cpu"
+ENV PIP_EXTRA_INDEX_URL=${PIP_EXTRA_INDEX_URL}
 RUN --mount=type=cache,target=/root/.cache/pip \
     --mount=type=bind,src=requirements-build.txt,target=requirements-build.txt \
     pip install --upgrade pip && \