make imagebuild logs independent of cluster locality

[geropl]

Context: #7435

Currently imagebuilds are delivered via server -> image-builder-mk3. This was the backwards-compatible way from mk2, but requires server and image-builder_mk3 to reside in the same cluster (= region).

With this issue the plan is to re-use the per-workspace log-streaming mechanism we already have in place for prebuilds. This enables:

• removing the last bits of cross-cluster messagebus
• the move of image-builder into the workspace cluster