Allow system Python discovery with `--target` and `--prefix` (#9371)
## Summary
If we're installing with `--target` or `--prefix`, then it's not a
mutable operation, so we should be allowed to discover system Pythons. I
suspect this was hard to special-case in the past but is now trivial
after @zanieb's various refactors.
Closes https://github.com/astral-sh/uv/issues/9356.
