Migrate Buildkite CI queues from AWS to GKE

[mstifflin]

What changed / Why:

• Update Buildkite pipeline yaml to work with the newly provisioned queues in Google Kubernetes Engine
• Use agent-stack-k8s v0.8.0 helm chart which has its own expected pipeline yaml syntax in order to successfully onboard.
• Remove the docker pre-build + push + wait steps in the pipeline
  • Simplifies the pipeline, no need to worry about push/pull permissions
  • No real impact on execution time
  • It's ok to do this duplicate work on the agents since builds are relatively infrequent
• Hard code the ssh volume mount name in docker-compose (complains of empty env var value otherwise)
• Downgrade pipeline docker compose version
  • Necessary since it will otherwise fail with /tmp/github-com-buildkite-plugins-docker-compose-buildkite-plugin/hooks/../commands/run.sh: line 99: BUILDKITE_AGENT_ID: unbound variable
  • Opened a thread with Buildkite support in Slack, but this pipeline isn't using any of the newer docker-compose plugin features so I don't think it needs to be on latest

Tested

• https://buildkite.com/uberopensource/yarpc-go/builds/5523

• [ ] Description and context for reviewers: one partner, one stranger

• [ ] Docs (package doc)

• [ ] Entry in CHANGELOG.md

[CLAassistant]

All committers have signed the CLA.