uv python install: remove the existing version only after the new installation is downloaded successfully (#8485)

## Summary

This PR delays the removal of an existing version after downloading the
new version when running `uv python install --reinstall`.

If the download fails, we can keep the existing version working.

## Test Plan

```console
$ cargo run -- python install 3.13
$ cargo run -- python install --reinstall 3.13 # when downloading, `ctrl-c` to interrupt
$ cargo run -- python list
```

crates/uv-python/src/installation.rs

@@ -132,7 +132,7 @@ impl PythonInstallation {
 
         info!("Fetching requested Python...");
         let result = download
-            .fetch(&client, installations_dir, &cache_dir, reporter)
+            .fetch(&client, installations_dir, &cache_dir, false, reporter)
             .await?;
 
         let path = match result {