Add optional GitHub API authentication headers

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

roles/llvm/tasks/Ubuntu.yaml

@@ -2,6 +2,7 @@
 - name: get latest stable github release
   uri:
     url: https://api.github.com/repos/llvm/llvm-project/releases/latest
+    headers: '{{github_auth_headers}}'
   register: llvm_latest
 
 - name: read /etc/os-release