Only run enable_first build commands the first time an env is enabled

https://github.com/terra-ops/terra-ui-prototype has 

```
drush {{alias}} site-install standard --db-url=mysql://drupal:drupal@database/drupal -y
```

in the `enable_first` section of .terra.yml.

Currently on my localhost, re-enabling an existing environment runs these commands and my Drupal db gets wiped.  Perhaps a flag isn't getting set properly somewhere?
