Preconditions are never met if creating project inside existing workspace

If you have an existing repo and you try to create a new project inside it, creating the project dirties the working tree such that subsequent add-ons will fail the preconditions check. Repro (make sure you select an add-on like `prettier`):

```bash
mkdir my-workspace
cd my-workspace
git init
npx sv create apps/my-app
```