feat(ci): add merge step to CI pipeline

[nijel]

Proposed changes

• this verifies that merging actually works before doing publish
• can be later used for proper merge as described in Avoid invoking build multiple times in CI pipeline #2589

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information