Add cleanup script for MR VM images

The following discussion from !1 (merged) should be addressed:

• @steveazz started a discussion: (+3 comments)

  suggestion: Is there a specific reason why we use 15 in this case?

  Would it make more sense to use the $CI_MERGE_REQUEST_ID here so we know which merge request created the base image? This also gives us the benefits that if we have concurrent merge requests they don't collide together?

      OUTPUT_IMAGE_NAME: runners-macos-dev-${CI_MERGE_REQUEST_ID}-mr.img