ci: Publish a multi-platform image once more
Now that T322453 has been resolved and the BuildKit frontend no longer
relies on dockerfile2llb
, a multi-platform image is possible and will
hopefully provide folks developing on arm64 a way to run Blubber builds
without emulation.
Bug: T318866 Change-Id: Ibe19b7faae99d8e531b90dbc72e5f7a2b884310a