Migrate supervisor builds away from legacy builder action

[sairon]

.github/workflows/builder.yml:208 and .github/workflows/builder.yml:262 still use home-assistant/builder@2026.02.1 for Supervisor image builds.

Migrate the release and test workflow paths to the composite builder actions from home-assistant/builder#273 while preserving the current arch matrix, signing, publish flow, and local runner test build behavior.

Done when:

• .github/workflows/builder.yml no longer references home-assistant/builder@
• the current release/publish flow still works
• the local runner test build path still works
• the generic multi-arch image/manifests expected by the epic are published