fix: Use GitHub App token for auto-fix commits to trigger CI

[Jeadie]

Summary

• Commits pushed by the lint-fix job using the default GITHUB_TOKEN don't trigger new CI workflow runs (GitHub's intentional loop prevention)
• Switch to a GitHub App installation token via actions/create-github-app-token@v1 so that auto-fix commits re-trigger required checks

Setup required

Before merging, a GitHub App must be configured:

1. Create a GitHub App at https://github.com/organizations/spiceai/settings/apps/new (or use an existing one)
   • Give it Contents: Read & write repository permission
   • Install it on the spicebench repository
2. Add repository variable BOT_APP_ID — the App's numeric ID (found on the App's settings page)
3. Add repository secret BOT_PRIVATE_KEY — the App's generated private key (PEM format)

Test plan

• Confirm BOT_APP_ID variable and BOT_PRIVATE_KEY secret are configured
• Open a PR with a minor formatting issue, verify the lint-fix job commits a fix and CI re-runs on the new commit