ci: fix failing dev server tests on PRs from forks (#8153)

#### Summary

On fork PRs, secrets can't be resolved, so `NETLIFY_AUTH_TOKEN` becomes
an empty string, because of the way this line was written:

```yaml
NETLIFY_AUTH_TOKEN: ${{ secrets.NETLIFY_AUTH_TOKEN }}
```

This causes some integration tests to fail ([example
job](https://github.com/netlify/cli/actions/runs/24239957758)) even
though they don't actually need a real token. Set a fallback
'fake-token' to fix this.

> [!TIP]
> I sent this PR from a fork to self-validate the fix.