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.
dpl-snap only supports pushing one snap at a time.
Instead of many repetitive deploy statements,
we use our own script to loop over the snaps to be released.