Change to show only the python installed on the system if `--python-preference only-system` is specified (#5219)
Fix #5211
## Summary
Change to show only the python installed on the system if
`--python-preference only-system` is specified.
Below is an example of running the command before the change, showing
Python not installed on the system.
#### Before
```bash
# Check system python
$ uv python --preview list --python-preference only-system
cpython-3.12.4-linux-x86_64-gnu <download available>
cpython-3.12.3-linux-x86_64-gnu /usr/bin/python3.12
cpython-3.12.3-linux-x86_64-gnu /usr/bin/python3
cpython-3.12.3-linux-x86_64-gnu /bin/python3.12
cpython-3.12.3-linux-x86_64-gnu /bin/python3
cpython-3.11.9-linux-x86_64-gnu <download available>
cpython-3.10.14-linux-x86_64-gnu <download available>
cpython-3.9.19-linux-x86_64-gnu <download available>
cpython-3.8.19-linux-x86_64-gnu <download available>
cpython-3.7.9-linux-x86_64-gnu <download available>
```
This PR changes the display to show only Python installed on the system.
#### After
```bash
$ cargo run python --preview list --python-preference only-system
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.11s
Running `target/debug/uv python list --python-preference only-system --preview`
cpython-3.12.3-linux-x86_64-gnu /usr/bin/python3.12
cpython-3.12.3-linux-x86_64-gnu /usr/bin/python3
cpython-3.12.3-linux-x86_64-gnu /bin/python3.12
cpython-3.12.3-linux-x86_64-gnu /bin/python3
```
## Test Plan
- `cargo run python list --python-preference only-system` in Ubuntu
24.04 to verify the display.
- `cargo run python list` in Ubuntu 24.04 to verify that the results
before and after the change were the same.