Satis bin is prompting for auth credentials when they are present in auth.json

Using the latest version from master, specifying vcs (git) over http, Satis is requesting the password when the credentials are present in `auth.json` in Composer's home.

Giving it the password, it succeeds but doesn't offer to install them in the auth.json file.

running -vvv is states to following 2 lines:
```
Loading config file /home/ubuntu/.composer/config.json
Loading config file /home/ubuntu/.composer/auth.json
```

Oddly, running `composer exec satis build satis.json public` it works fine, albeit there's not Stdout. Suggesting there's an issue with the way the satis binary is reading the auth file?