Convert reusable-build-image.yml: setup-runner, chunka, push-image

[castrojo]

Part of epic #1420

Replace inline scaffolding steps with shared action calls.

Changes

1. setup-runner (lines 116–132)

Replace inline podman update from resolute with:

- uses: projectbluefin/actions/bootc-build/setup-runner@<SHA> # v1
  with:
    storage-backend: remove-software
    update-podman: true
    install-tools: '[]'

Note: LTS currently runs maximize-build-space (ublue-os/remove-unwanted-software) separately at line 134. Consolidate into setup-runner.

2. chunka rechunk (lines 211–242)

Replace inline chunkah build with:

- name: Rechunk image
  if: inputs.rechunk && inputs.publish
  uses: projectbluefin/actions/bootc-build/chunka@<SHA> # v1
  with:
    source-image: localhost/${{ env.IMAGE_NAME }}:${{ env.DEFAULT_TAG }}
    max-layers: '128'

This eliminates the inline CHUNKAH_REF pin, buildah build command, and save/load pipe — all handled inside the action with the same workarounds already documented there.

3. push-image (lines 267–301)

Replace inline push loop with:

- id: push
  uses: projectbluefin/actions/bootc-build/push-image@<SHA> # v1
  with:
    image-name: ${{ env.IMAGE_NAME }}
    tags: ${{ env.DEFAULT_TAG }}-${{ matrix.platform }}
    default-tag: ${{ env.DEFAULT_TAG }}-${{ matrix.platform }}
    force-compression: true    # gzip → zstd:chunked migration
    github-token: ${{ secrets.GITHUB_TOKEN }}

Prerequisite: actions#7 must land first (force-compression input).

Acceptance criteria

• All three action calls produce identical outputs to current inline steps
• Chunkah version stays in sync with the chunka action (not hardcoded in this workflow)
• CI passes on both main and lts branches