Add env UV_SYSTEM_PYTHON as alias to --system

[edenhaus]

Summary

Add a new env variable UV_SYSTEM as alias for the cli argument
--system.
Use cases:

• No need to specify on each uv call inside the docker container the
  --system flag
• It allows installing and configuring uv in a base container and in the
  child containers nobody needs to know if you need the --system cli
  flag
• The Home Assistant development env can be set up via devcontainer or a
  venv. Both use some common scripts. Instead of adding duplicate or
  special code to identify the dev container to set the --system flag,
  it would be nicer to set it via an env variable.

I'm unfamiliar with Rust and tried to add the support by looking at the
code.

Test Plan

I did test it manually
UV_SYSTEM_PYTHON=true uv pip install requests

[charliermarsh]

Thanks! I changed it to UV_SYSTEM_PYTHON, since UV_SYSTEM felt a little too generic. I know it deviates from the CLI but I think it's worth it.