This commit upgrades Github Actions workers to ubuntu-22.04
As part of that change, we (currently) no longer need the backports
nor toolchain-r/test PPAs, because ubuntu-22.04 include
recent-enough version of QEMU and gcc
As of the Clang 13 upgrade, we only need to build the toolchain once and
can use that toolchain for both x86_64 and i686. To do this, this breaks
the main Azure configuration into 3 "stages" (Lagom, Toolchain, and
Serenity), where the Serenity stage depends on the Toolchain stage.
This has the added benefit of uploading a new prebuilt toolchain cache
sooner than before, which should help alleviate pressure from PRs.
