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 master in 8 minutes and 23 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
manual #22026
allowed to fail manual
build:arch
passed #22025
build:bionic

00:08:22

passed #22024
build:buster

00:07:06

passed #22023
build:stretch

00:07:20