bugfix: pip_install_packages="" now doesn't break sandbox

[snimu]

Description

In vf.PythonEnv, the pip_install_packages argument causes Sandboxes to fail. Due to the error catching, this is only visible in the tool-outputs. The model still continues working (and succeeds in many environments like math-python).

The error is caused by the _START_COMMAND_TEMPLATE having pip install -q {pip_install_packages} in it, which errors when pip_install_packages = "". This is now replaced with {pip_install_command}, which in turn is guaranteed to be empty if there are no packages to install.

Previously, running uv run vf-eval math-python -n 1 -r 1 -m gpt-5-mini -a '{"pip_install_packages": ""}' resulted in outputs like this, which resulted in full reward and a functional rollout, just no usable sandboxes:

Now, the sandboxes work (though the models still assume the presence of numpy, scipy, and sympy for math-python; but that's not an issue with the sandboxes):

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Test improvement

Testing

• All existing tests pass when running uv run pytest locally.
• New tests have been added to cover the changes

Checklist

• My code follows the style guidelines of this project as outlined in AGENTS.md
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Additional Notes