Avoid comparing to system site packages in `--dry-run` mode (#11427)
## Summary
Right now, `uv sync --dry-run` returns the interpreter that _would've_
been used to create the environment; so we end up using _that_
interpreter's `site-packages`, and return changes relative to that
interpreter. Instead, we now create a temporary virtual environment and
compare against that.
Closes https://github.com/astral-sh/uv/issues/11422.
