Add --local flag

[morags]

Feature request

Add a flag that forces uv to work completely locally, with no changes to the global environment.

Background

I have two use cases for which a completely local installation (no global python, no global cache, perhaps not even a global uv) is needed:

1. Distributing packages to systems that don't have, or can't have Python installed (see eg. here).
2. Installing a package that is sensitive to non-ASCII paths (eg. C:\Users\Иван\AppData\Roaming\uv\)

At the moment there are several flags and environment variables that allows you to control parts of this behavior, like --cache-dir and UV_PYTHON_INSTALL_DIR, but it would be useful to have a single flag that concentrates all of them and forces uv to work in a completely isolated fashion.

Suggested behavior

uv <command> --local <folder> runs a command in a given folder without touching the global environment, including caches, Python installations and configuration files.