Allow sync to install to system python

[nhumrich]

In docker environments, it's quite common to install all python packages to the system. Virtual environments are not needed because the current project is the only thing installed.

The docker documentation for UV recommends that you use the venv entrypoint for all container commands:

CMD ["/app/.venv/bin/python"]

However, this makes migrating to UV a bit harder when so many things already reference "python" directly in.

Also, using a virtual environment in these situations complicates the container, because devs now have to remember to use the .venv python always.

Would it be possible to install into the system during uv sync somehow. I assumed --python-preference only-system would do this, but that doesn't seem to prevent the creating of a virtual environment. uv pip install support --system, so its sort of strange that sync does not.

My current workaround is doing this in the dockerfile instead:

RUN --mount=from=ghcr.io/astral-sh/uv,source=/uv,target=/bin/uv \
    uv pip compile pyproject.toml -o requirements.txt && \
    uv pip install -r requirements.txt --system

[zanieb]

See UV_PROJECT_ENVIRONMENT and #6834