pyproject : add linux platform marker to uv torch source (#29177)

In tool.uv.sources, torch was unconditionally pinned to the custom
pytorch CPU index, which lacks macOS Darwin wheels and causes uv sync
to fail on macOS. Add the sys_platform == 'linux' marker to match the
existing Poetry dependencies configuration.

Assisted-by: Antigravity

Resolves: https://github.com/ggml-org/llama.cpp/issues/29176