[fix] prevent macos-13 runner starvation from blocking docker image update

[hello-stephen]

Problem

The `build` matrix job includes `macOS-x86_64` running on `macos-13`. GitHub Actions `macos-13` (Intel Mac) runners have become increasingly scarce and can queue for up to 24 hours before timing out. This caused a cascade of failures:

1. `macos-13` runner waits 24h → times out → entire `build` matrix result = `failure`
2. `update-docker` condition requires `build.result == 'success'` → skipped
3. `success` job depends on `update-docker` → skipped
4. `failure` job has `if: failure()` but `update-docker` was skipped (not failed), so `failure` also does not run
5. Release status is stuck at `BUILDING` indefinitely; Docker image is never updated

This was observed in run #26616785582 where the `macOS-x86_64` build waited exactly 24h0m0s.

Fix

`build-4.0.yml`

• Remove `macOS-x86_64` (macos-13) from the matrix entirely — branch-4.0 has no downstream consumers that need Intel Mac artifacts; Apple Silicon covers all active Mac use cases.
• `fail-fast: false` so a future matrix failure does not cancel other in-progress builds.
• `if: always() && failure()` on the `failure` job — fires even when `update-docker` is skipped rather than failed, preventing status from being stuck at `BUILDING`.

`build.yml`

• Keep `macOS-x86_64` but add `continue-on-error: true` — master branch still ships Intel Mac artifacts when runners are available, but a timeout no longer blocks `update-docker`.
• `fail-fast: false` so a macOS timeout does not cancel the arm64/Linux builds.
• `if: always() && failure()` on the `failure` job — same as above.