Disable jekyll-github-metadata gem when building locally

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (either `gem update github-pages` or `bundle update github-pages`)

### This issue affects

- [ ] The site generated by GitHub Pages
- [x] Building sites locally

### What did you do (e.g., steps to reproduce)
Run `bundle exec jekyll serve` from a fresh github-pages repo.

### What did you expect to happen?
All build smoothly.

### What happened instead?
`GitHub Metadata: No GitHub API authentication could be found. Some fields may be missing or have incorrect data.`

### Additional information
The github-pages project I'm trying to setup does not need any metadata from my github repository. Is there a way to disable `jekyll-github-metadata` gem when building locally?

PS: I'm aware of the authentication configuration needed for `jekyll-github-metadata` gem, but since I'm not going to use any of those metadata, it will be awful if I have to do so.

Beginne damit, das Problem mit `bundle exec jekyll serve` in einem frischen github-pages-Repository zu reproduzieren, und untersuche, wie der lokale Jekyll-Build `jekyll-github-metadata` lädt. Als erledigt gilt die Aufgabe, wenn lokale Builds abgeschlossen werden, ohne eine GitHub-API-Authentifizierung zu erfordern, sofern keine Repository-Metadaten verwendet werden; das Issue nennt keine bestimmte Datei oder keinen bestimmten Test.

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