[Feature Request] Support the `--isolated` flag for pip

[TalAmuyal]

First, thanks for investing in Python's ecosystem!

Context:

I'm trying to incorporate uv into the flow of our devs at my company.

From the output of python -m pip --help:

  --isolated           Run pip in an isolated mode, ignoring environment variables and user configuration.

The request:

I would imagine that uv pip --isolated ... will only impact the pip "environment variables and user configuration" and won't change behavior for uv's specifics.

For example, it will cause uv pip install to ignore PIP_INDEX_URL, but not VIRTUAL_ENV.

I hope it makes sense.

Thanks!