fix: llama stack build use UV_SYSTEM_PYTHON to install dependencies to system environment (#1163)

# What does this PR do?
- resolves issue: #1159 
- Root cause: https://github.com/meta-llama/llama-stack/pull/980 forces
`build_venv.sh` to install in a venv environment, which do not work on
Colab notebook environment

<img width="1004" alt="image"
src="https://github.com/user-attachments/assets/1f9be409-5313-4926-b078-74e141cf29eb"
/>

## This PR
Use `UV_SYSTEM_PYTHON` to make sure dependencies are installed in
current system environment. Which will be used in the Colab environment.
```
UV_SYSTEM_PYTHON=1 llama stack build --template together --image-type venv
```

## Test Plan
- Works in Colab environment
<img width="621" alt="image"
src="https://github.com/user-attachments/assets/ae93bc3d-e05a-44b9-bb21-fb88f29969b8"
/>

llama_stack/distribution/build_venv.sh

@@ -74,10 +74,15 @@ run() {
   local pip_dependencies="$2"
   local special_pip_deps="$3"
   
+  if [ -n "${UV_SYSTEM_PYTHON:-}" ]; then 
+    echo "Installing dependencies in system Python environment"
+  else
     echo "Using virtual environment $env_name"
     uv venv "$env_name"
     # shellcheck source=/dev/null
     source "$env_name/bin/activate"
+  fi
+
   if [ -n "$TEST_PYPI_VERSION" ]; then
     # these packages are damaged in test-pypi, so install them first
     uv pip install fastapi libcst