bootstrap: Backup settings.json to the correct filename

The old code actually replaced `.json` with `.bak` (so, `settings.bak`),
rather than appending `.bak` as claimed (`settings.json.bak`).

`Path::set_extension` can instead be used with dots:

> The new extension may contain dots and will be used in its entirety,
> but only the part after the final dot will be reflected in
> self.extension.

src/bootstrap/setup.rs

@@ -582,7 +582,7 @@ fn create_vscode_settings_maybe(config: &Config) -> io::Result<()> {
             Some(false) => {
                 // exists and is not current version or outdated, so back it up
                 let mut backup = vscode_settings.clone();
-                backup.set_extension("bak");
+                backup.set_extension("json.bak");
                 eprintln!("warning: copying `settings.json` to `settings.json.bak`");
                 fs::copy(&vscode_settings, &backup)?;
                 "Updated"