Make parts of the Buildkite pipeline public

Created by: valerybugakov

Context

The problem

Outside contributions to sourcegraph/sourcegraph or other repos where we run tests in Buildkite are a major security concern. Our Buildkite pods contain critical secrets as environment variables that could be exfiltrated by a malicious contribution. This includes GCP service account creds, GitHub API keys and more. It would be very similar to the Codecov incident.

Slack thread.

Current solution

The Buildkite CI is disabled for forks.

Success criteria

Steps that do not require any secrets should be made public.