Refactor CI/CD config

Summary

Refactor CI/CD config to improve readability and maintainability.

• build job extends ci job. Move shared script to before_script.
• Single build job using https://docs.gitlab.com/ci/yaml/#parallelmatrix.
• Make GOOS and GOARCH CI variables, always set GOOS explicitly

Verification

• Verify branch pipelines (ci job).
  • See https://gitlab.com/gitlab-org/govern/secrets-management/openbao-internal/-/jobs/11684431371
• Verify pipelines for git tags (build job).
  • See !27 (comment 2814585172)
  • Build jobs have been canceled to avoid pushing to the package registry.
  • To be double checked after when creating the first git tag after merging.

Next steps

• Rename ci job. It runs make check really.
• Rename ci stage. It looks like this is test stage.