When cloning a new project from the examples dir via create-payload-app,
the corresponding `.env` file is not being generated. This is because
the `--example` flag does not prompt for database credentials, which
ultimately skips this step.
For example:
```bash
npx create-payload-app --example live-preview
```
The result will include the provided `.env.example`, but lacks a `.env`.
We were previously writing to the `.env.example` file, which is
unexpected. We should only be writing to the `.env` file itself. To do
this, we only write the `.env.example` to memory as needed, instead of
the file system.
This PR also simplifies the logic needed to set default vars, and
improves the testing coverage overall.
836fd86090