Always treat conda environments named `base` and `root` as base environments (#15682)
Extends https://github.com/astral-sh/uv/pull/15679
I'm not sure if we want to do this. It is probably _generally_ true, but
I think I'd rather remove the special casing entirely? I think the main
case for this is that it could help prevent regressions from #15679 and
we can remove it in a breaking release?