ci: Build for both linux/amd64 and linux/arm64

Now that Blubber (and Kokkuri) support targeting multiple architectures
and publishing under a single manifest index, let's publish versions
that work on both linux/amd64 and linux/arm64.

Bug: T318866
3 jobs for !36 with review/multi-target in 9 seconds (queued for 9 seconds)
latest merge request