Fix uv pip compile with UV_SYSTEM_PYTHON=1

[jfcherng]

Summary

Following up

• Use BoolishValueParser for boolean environment variables #3113
• Allow --python and --system on pip compile #3115

It looks like uv pip compile command with UV_SYSTEM_PYTHON=1 is missed because these two PRs are close in time. And thus resulting in

$ uv --version
uv 0.1.34 (9259eceeb 2024-04-19)

$ UV_SYSTEM_PYTHON=1 uv pip compile --upgrade requirements.in -o requirements.txt
error: invalid value '1' for '--system'
  [possible values: true, false]

For more information, try '--help'.

[charliermarsh]

Thx