setup_env.py triggers unwanted pip install, causing environment conflicts

从 setup_env.py 开始，阅读 main() 和 setup_gguf()，然后通过运行 python setup_env.py，在自定义 Python 虚拟环境中复现该行为。定义对 gguf-py 安装所采用的处理方式，并验证脚本不再执行不需要的无条件安装，同时为需要该依赖项的用户保留清晰的途径。

Bug Summary

The setup_env.py script breaks custom or virtual Python environments due to a forced pip installation. In the main() function, the very first step is to call setup_gguf(), which, in turn, runs:

sys.executable -m pip install 3rdparty/llama.cpp/gguf-py

This unconditional install step clashes with existing venv configurations, often causing dependency/version conflicts and making integration with user-managed Python environments difficult or unstable.

Steps to Reproduce

1. Set up a Python virtual environment with desired/compatible package versions.
2. Run python setup_env.py.
3. Observe that the script performs its own install step, overriding or conflicting with the current environment state.

Expected Behavior

The script should respect the user's Python environment and avoid running recursive or unconditional pip installs. It should either:

• Detect when dependencies are missing and prompt the user,
• Allow the user to opt-in or skip the pip install, or
• Provide clear documentation/instructions for manual installation.

Actual Behavior

• The script forces an install of its dependencies, potentially breaking user setups and overwriting targeted package versions.

Additional Context

This behavior makes it hard to use the script in controlled environments (e.g., CI pipelines, docker, or custom venvs) and introduces risk for version incompatibility bugs.

Suggested Fix:

• Make the install step optional, detectable by a command-line flag (e.g., --install-gguf)
• Or, remove the call from main() and only document the install as a manual, one-time user step.