If the `GITHUB_TOKEN` is defined in the Ansible controller's
environment it is used to set the GitHub REST API authentication header.
This is they passed to all `uri` modules tasks which interact with the
GitHub REST API. If the `GITHUB_TOKEN` is not set, the authentication
header is not used.
Fixes#19
Fixes#16 by replacing uses of the `apt_key:` module with `get_url:` to
download apt keyrings into `/etc/apt/keyrings`, then used
`signed-by=/etc/path/keyrings/<keyring>` in the appropriate sources.list
file.
