[FAST_BUILD] Merge manifests without pulling images

[mathbunnyru]

Describe your changes

Pros:

• works faster (as it pushes tag separately for 2 archs)
• design is slightly better - we should as much as possible in parallel
• doesn't use docker manifest, which is experimental
• less likely to timeout / run out of space
• The merge script only uses information from the registry and correctly handles some cases, where tag doesn't exist in the registry

Cons:

• duplication due to reusable workflows
• more workflows

Issue ticket if applicable

Checklist (especially for first-time contributors)

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests
• I will try not to use force-push to make the review process easier for reviewers
• I have updated the documentation for significant changes