feat(ci): skip setup-node and container when using self-hosted runner

[PierreBrisorgueil]

Context

Self-hosted ARC runners now use a custom image (ghcr.io/comes-io/arc-runner) with Node LTS pre-installed. The setup-node step and container: node:lts are redundant on self-hosted.

Changes needed

1. Remove container: node:lts — the runner IS the container
2. Conditionally skip setup-node:

- uses: actions/setup-node@v4
  if: ${{ !vars.RUNNER }}
  with:
    node-version: lts/*
    cache: npm

3. Keep services: mongo as fallback for GitHub-hosted, but self-hosted uses MONGO_URI variable pointing to dedicated mongo-test pod

Compatibility

• GitHub-hosted (vars.RUNNER not set): setup-node runs, services.mongo used — same as today
• Self-hosted (vars.RUNNER set): setup-node skipped, MONGO_URI overrides mongo host