With arm64-graviton2 now providing full VM virtualization, LXD containers
can be used to build snaps on all architectures.
This is beneficial for us as having snapcraft manage the whole build
environment is likely to yield better/more consistent results than
running the builds on the Travis images directly.
Snapcraft's remote-build does not fit the requirements of CI,
so replace it with builds running directly on Travis:
1. Builds on Travis can run independently, whereas remote-build
competes for the same resource (Launchpad) and will potentially
block if multiple commits require building in succession.
2. Snapcrafts CLI for remote-build is not designed to be easily
scriptable.
3. Travis recently introduced building on arm64, so builds for
arm64 are now very fast and don't require emulation.
We do not build for armhf and i386 any more because they are
of little relevance on the desktop.
We invoke builds on Launchpad in stage "deploy" and release it into
the edge channel of the Snap Store.
The deploy stage is blocked on fails of the test stage, so we don't
release snaps with failing tests.
