Stop using packages.tgz to get prebuilt packages

- Packages are now in apt repositories, so stop trying to get
packages.tgz from the previous continuous build if a user wants prebuilt
packages, simply fetching from apt instead. In the case of building
packages locally, we do still follow the same process as always.
- Also remove exporting of packages.tgz from several Kokoro builds, as
the artifact will no longer see any use.

Change-Id: I68be5a70bc47707498ec30fe6e1731256caaea73
8 files changed