apt: Use pinning instead of Apt::Default-Release

Setting a default release is useful when two archives (e.g. stable and
testing) are configured, but once a third archive (e.g. unstable) is
added, this gets messy. All no-default archives get chunked together
with the same priority, so once you install a version from any
non-default archive, it will happily upgrade to the most recent version
from another archive.

By giving each archive a separate priority, this shouldn't happen. Since
the version numbers in the archives typically increment in the reverse
order of the priorities, this should do the right thing in most
circumstances.