apt.llvm.org signature has changed, we also should use debian:stable
(=buster) as a base image to correctly install packages.
Removed jdk from the base image as it was used only by Jenkins.
Log packages version for every build.
Related issues: #112
- configured sccache on buildkite windows machines
- final result is sent to phabricator from "summary.py" that waits for both builds to complete
- extracted "add_url_artifact" to a runnable script
- reorganized code and fixed some of TODOs
