Fix system site packages detection default

[konstin]

See https://docs.python.org/3/library/site.html and https://github.com/python/cpython/blob/a549f439384b4509b25639337ffea21c2e55d452/Lib/site.py#L614-L639: The default is for system site packages is true, they have to be disabled by setting the key to something that is not true.

[zanieb]

Do we need to change

uv/crates/uv-cli/src/lib.rs

Lines 2387 to 2395 in acbbb2b

	/// Give the virtual environment access to the system site packages directory.
	///
	/// Unlike `pip`, when a virtual environment is created with `--system-site-packages`, uv will
	/// _not_ take system site packages into account when running commands like `uv pip list` or `uv
	/// pip install`. The `--system-site-packages` flag will provide the virtual environment with
	/// access to the system site packages directory at runtime, but will not affect the behavior of
	/// uv commands.
	#[arg(long)]
	pub system_site_packages: bool,

[charliermarsh]

I don't think so. This just affects what value we infer if nothing is set, but we still set it to false by default in uv venv.