build/bake: add reusable workflow job name prefix

[crazy-max]

relates to https://github.com/docker/packaging/actions/runs/28931486137/job/85831553565

run
├─ prepare
├─ prepare
├─ build (0, linux/amd64, ubuntu-24.04)
├─ build (1, linux/arm64, ubuntu-24.04-arm)
├─ finalize
├─ prepare
├─ build (0, linux/amd64, ubuntu-24.04)
├─ build (1, linux/arm64, ubuntu-24.04-arm)
├─ finalize
├─ prepare
├─ build (0, linux/amd64, ubuntu-24.04)
├─ build (1, linux/arm64, ubuntu-24.04-arm)
├─ finalize
├─ prepare
├─ build (0, linux/amd64, ubuntu-24.04)
├─ build (1, linux/arm64, ubuntu-24.04-arm)
├─ finalize
├─ prepare
├─ build (0, linux/amd64, ubuntu-24.04)
├─ build (1, linux/arm64, ubuntu-24.04-arm)
├─ finalize
├─ prepare
├─ build (0, linux/amd64, ubuntu-24.04)
├─ build (1, linux/arm/v7, ubuntu-24.04-arm)
├─ build (2, linux/arm64, ubuntu-24.04-arm)
└─ finalize

This adds an optional job-name-prefix input to the build and bake reusable workflows so callers can disambiguate nested reusable workflow jobs in the GitHub Actions UI.

The build and bake workflows now use the prefix when naming their internal prepare, build, and finalize jobs, while preserving the existing job names when the input is not set.

GitHub Actions flattens jobs from nested reusable workflows, which makes repeated prepare, build, and finalize jobs hard to scan when a caller fans out across many targets. This keeps the default behavior stable while giving callers a simple opt-in way to make those jobs readable.