Build the auto-hardfork image inside AutoHardforkTest

`AutoHardforkTest` depended on the packaging job's `daemon_auto_hardfork` docker
step, so every PR touching `src/` pulled the whole Debian + Docker artifact
build in as a dependency.

The job now packages the two debs the image installs (`daemon_devnet_postfork`,
`logproc`) from the app-build binaries and builds the image locally with
`--load-only` — no cached debians, and no image push. The legacy prefork deb
still comes from the cache's `legacy/debians` root, as it is a released
pre-fork artifact.
