[MMI] Adds mmi build step

[zone-live]

This adds the prep-build-mmi step, it's necessary and convenient for us to have the artefact generated, it also validates that any change introduced breaks the build.
CircleCI run

Pre-merge author checklist

• I've clearly explained:
  • What problem this PR is solving
  • How this problem was solved
  • How reviewers can test my changes
• Sufficient automated test coverage has been added

Pre-merge reviewer checklist

• Manual testing (e.g. pull and build branch, run in browser, test code being changed)
• IF this PR fixes a bug in the release milestone, add this PR to the release milestone

If further QA is required (e.g. new feature, complex testing steps, large refactor), add the Extension QA Board label.

In this case, a QA Engineer approval will be be required.

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[codecov]

Codecov Report

Merging #20207 (90d5c2e) into develop (d262acb) will not change coverage.
Report is 14 commits behind head on develop.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop   #20207   +/-   ##
========================================
  Coverage    68.70%   68.70%           
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  Files          986      986           
  Lines        37817    37817           
  Branches     10116    10116           
========================================
  Hits         25979    25979           
  Misses       11838    11838