Add GitHub Actions configuration
This PR adds the GitHub Actions configuration to the rust-lang/rust repository. The configuration will be run in parallel with Azure Pipelines until the evaluation finishes: the infrastructure team will then decide whether to switch.
Since GitHub Actions doesn't currently have any way to include pieces of configuration, this also adds the `src/tools/expand-yaml-anchors` tool, which serves as a sort of templating system. Otherwise the configuration is a mostly straight port from the Azure Pipelines configuration (thanks to all the PRs opened in the past).
There are still a few small things I need to fix before we can land this, but it's mostly complete and ready for an initial review.
r? @Mark-Simulacrum
2dcf54f564