fix(ci): switch apt mirror per runner — azure on github-hosted, kernel.org on self-hosted

Self-hosted runners (arc-runner-set, bigger-runner) cannot reach
azure.archive.ubuntu.com — they live in different networks (e.g. our
arc-runner-set Kubernetes cluster) where Azure's mirror IP is not
routable. Symptom: "Connection failed [IP: 51.11.236.225 80]" with each
Ign:/Err: cycle taking 60s, hanging the build for ~16 minutes before
exit 100.

Pick the mirror based on `runner.environment`:

  * github-hosted (ubuntu-latest, ubuntu-24.04-arm) → Azure
    (http://azure.archive.ubuntu.com / http://azure.ports.ubuntu.com)
    — same VPC as the runner.
  * self-hosted (arc-runner-set, bigger-runner)    → kernel.org
    (https://mirrors.edge.kernel.org for both archive and ports)
    — publicly reachable from any network.

The choice now lives in one place: the .github/actions/configure-apt-mirror
composite action exposes `effective-mirror` / `effective-ports-mirror`
outputs so the reusable workflows can forward the same value as Docker
build-args without duplicating the per-runner-environment branch.

The now-redundant `apt-mirror` / `apt-ports-mirror` workflow inputs on
image_build.yml and backend_build.yml are dropped — defaults live in the
composite action and are visible there.

Assisted-by: Claude:claude-opus-4-7[1m] [Claude Code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>