fix: corrected allcontributors setup import path with fs

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🐛 Bug: Setup script crashes with [ERR_MODULE_NOT_FOUND] for .all-contributorsrc.json #117
• That issue was marked as accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Turns out the file path that's being imported is .allcontributorsrc, not .allcontributorsrc.json. ES Module JSON imports don't support importing files that don't have the .json extension. 🤷

So this switches to using JSON.parse and fs.readFile.