CI: Codify how to build on Arch Linux

Some libcapsule users and contributors are using Arch Linux or Manjaro
rather than a Debian derivative.

Many of the tests will be skipped on Gitlab-CI because they need a
working bubblewrap, which isn't allowed inside unprivileged Docker;
but this provides "executable documentation" for how to do a build
and test.

Signed-off-by: Simon McVittie <smcv@collabora.com>
4 jobs for wip/arch in 8 minutes and 20 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
manual build:arch #22022
allowed to fail manual
passed build:bionic #22021

00:08:05

passed build:buster #22020

00:07:15

passed build:stretch #22019

00:08:20