Parse source commit and branch in preflight builds

[mcncl]

Description

Supports the passing of BUILDKITE_PREFLIGHT_SOURCE_COMMIT to preflight builds, as well as BUILDKITE_PREFLIGHT_SOURCE_BRANCH when not in a detached HEAD state.

Changes

• Resolves the git context from the original state that a preflight build was created from
• Expands the build env to always include BUILDKITE_PREFLIGHT_SOURCE_COMMIT in preflight builds
• When not in detached HEAD, we will also set BUILDKITE_PREFLIGHT_SOURCE_BRANCH, where the source branch can be resolved

Testing

• Tests have run locally (with go test ./...)
• Code is formatted (with go fmt ./...)