Add the `--active` flag that will choose whatever environment that is currently active

[lwasser]

There's also the --active flag that will choose whatever environment that is currently active

Originally posted by @yeelauren in #584 (comment)

[lwasser]

When we talk about UV here:
https://www.pyopensci.org/python-package-guide/package-structure-code/declare-dependencies.html#combining-dependency-groups
in the guide, we don't mention the --active flag that you can use to ensure you are using the active environment with uv pip install it would be good to add that to the guide. this is a small update to the page that someone who is familiar with uv and the active flag can easily complete quickly!