Make `TF_IN_AUTOMATION` env var for terraform commands optional

### Describe the Feature

Terraform does not check the value of `TF_IN_AUTOMATION`, only that it is set. As soon as it's set, the behavior takes effect. We hard code this setting, and don't provide a way to unset it.

### Expected Behavior

`TF_IN_AUTOMATION` should be optional.

### Use Case

When doing a lot of development with atmos, sometimes it's convenient to also use native `terraform` commands. If `TF_IN_AUTOMATION` is set, then it won't emit the `terraform` command you can run to apply it.

### Describe Ideal Solution

Can we simply rely on setting the following in `atmos.yml`, so that users can change the behavior.

```
env:
TF_IN_AUTOMATION: true
```

### Alternatives Considered

_No response_

### Additional Context

_No response_