Move pip install into dedicated builder stage

[lightswitch05]

Why

#343 introduced isolated builder stages for ssocr, libcec, PicoTTS, and Telldus, using COPY --link to merge them into the final image. The --link flag decouples each COPY from the parent layer chain, so a change to the base image or any other stage does not invalidate the cached result of unrelated stages — each builder is independently reusable across builds.

That PR also attempted a pip-install-builder stage for Python packages, but it used pip install --user, which puts packages in /root/.local/ rather than /usr/local/lib/python*/site-packages/ where Home Assistant expects them. That part was reverted in #358.

#401 migrated the workflow to the new build-image action, which uses BuildKit with mode=max layer caching backed by GitHub Actions cache. This is what actually makes the multi-stage cache reuse from #343 pay off at CI time — every builder stage's layers are captured and reused across workflow runs.

What

Adds a pip-builder stage that installs Python packages using --prefix=/opt/pip-packages (instead of --user), then COPY --links the result into /usr/local/ in the final image — landing packages at /usr/local/lib/python*/site-packages/ exactly where they were before.

This follows the same pattern already used for ssocr, libcec, PicoTTS, and Telldus. With this in place, a requirements.txt change triggers only the pip-builder stage; ssocr, libcec, picotts, and telldus remain cache hits. Equally, a version bump in any of those builders leaves the pip layer untouched.

Also makes cache-gha: true and cache-image-tag: latest explicit in the builder workflow, so caching intent is visible rather than implied by action defaults.

[lightswitch05]

I just realized why cache isn't working like I had thought with the original PR. FROM ${BUILD_FROM} is used for all the builder stages. If base changes (which is the main reason to re-build this image) - then all the builder stage caches are thrown away too. With that realization, I don't think this PR adds any value.