Fix Conda Python detection on Windows (#2279)
## Summary
In #2102, I did some refactor, and changed a method to return the Python
executable path rather than the parent directory path. But I missed this
one codepath for Conda on Windows.
Closes https://github.com/astral-sh/uv/issues/2269.
## Test Plan
- Installed micromamba on my Windows machine.
- Reproduced the failure in the linked issue.
- Verified that `python.exe` exists at `${CONDA_PREFIX}\python.exe`.
- Ran with this change; installed successfully.
