feat(ci): setup GitHub Action runners for building Ruby from source #225
Description
Problem
Some Ruby versions (currently 3.2.10, 3.4.8, 4.0.0, 4.0.1) only have Windows builds in our manifest because ruby/ruby-builder's toolcache release hasn't published darwin/linux builds for them. This causes integration test failures for "Migrate Ruby from System (macOS)".
Proposed Solution
Create a GitHub Actions workflow that uses ruby-build (rbenv's build tool) to compile Ruby from source for missing platform/version combinations and upload the resulting binaries to R2.
Scope
- Build Ruby from source on macOS (arm64, amd64) and Linux (arm64, amd64) runners
- Upload built binaries to R2 with proper metadata
- Trigger when gaps are detected in manifests after mirror sync
- Only build versions that are missing from specific platforms