fix: use auth.stdout, not .toString(), in setup (#389)

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## PR Checklist

- [x] Addresses an existing open issue: fixes #388
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/template-typescript-node-package/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/template-typescript-node-package/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

Missed in the `execa` conversion (#349), and not tested because it
involves directly using the GitHub API.

Author: JoshuaKGoldberg

script/setup.js

@@ -390,7 +390,7 @@ try {
 		await withSpinner(
 			async () => {
 				await $`gh auth status`;
-				const auth = (await $`gh auth token`).toString().trim();
+				const auth = (await $`gh auth token`).stdout.trim();
 
 				octokit = new Octokit({ auth });
 			},