Build the debian tests' packages in the test job

`DebianUpgradeTest` and `DebianAutomodeTransitionTest` were the last jobs
pulling `MinaArtifactBullseye` into a source PR: they need a handful of `.deb`
files, and depending on the packaging job also builds every docker image.

Both now package what they install from the app-build binaries and depend only
on the apps build. Where the debs come from is a new shared helper,
`buildkite/scripts/debian/fetch_debs.sh`: `LOCAL_DEB_SOURCE_DIR` selects debs
the job just built, otherwise it reads the build cache exactly as before, so the
nightly-only mainnet variant is unaffected. Prefork packages always come from
the legacy cache root.

With this, an ordinary source PR builds no debian packages and no docker images.
