Each architecture-specific build job was pushing tags for BOTH amd64 and arm64,
causing the last-finished build to overwrite the other's tags. This resulted in
users pulling arm64-tagged images but receiving amd64 binaries, leading to
'exec format error' on ARM64 hosts.
Fix: Each job now only pushes tags with its own architecture suffix:
- build-amd64 → only -amd64 tags
- build-arm64 → only -arm64 tags
The create-manifest job correctly combines both into multi-arch manifests
with clean (no suffix) tags.
Fixes#4566
