Add env UV_SYSTEM as alias to --system (#2354)
## 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`
