Invert the logic for determining if a path is a base conda environment (#15679)
Closes https://github.com/astral-sh/uv/issues/15604
The previous logic does not match the discussion in the original issue
about this feature, nor does it match the comment for the function. I'm
confused because I know this logic is working for some people? I'm
consequently a little wary of making this change. I'm following up with
some additional changes that should ensure this is robust, e.g., #15680