Fix `SourceNotAllowed` error message (#13012)
## Summary
Before:
```console
$ uv python list py --managed-python
error: Interpreter discovery for `executable name `py`` requires `search path` but only only managed is allowed
```
After:
```console
$ uv python list py --managed-python
error: Interpreter discovery for `executable name `py`` requires `search path` but only `only managed` is allowed
```
