Test flatpak build

This makes sure the flatpak build actually works and we have
the examples available for easy download.

This is somewhat based on the GTK+4 flatpak CI test.

(cherry picked from commit bcd03848)
12 jobs for ci-backport in 17 minutes and 40 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed api-visibility #240137
librem5

00:03:13

passed build-debian-clang #240140
librem5

00:04:33

passed build-debian-gcc #240138
librem5

00:03:47

passed build-debian-gcc-static #240139
librem5

00:03:44

passed build-fedora-gcc #240141
librem5

00:02:54

 
  Test+Docs
passed build-gtkdoc #240142
librem5

00:02:27

passed unit-test #240143
librem5

00:02:23

44.5%
 
  Package
passed package-debian-buster #240144
librem5

00:02:46

passed package-debian-buster:arm64 #240145
librem5:arm64

00:06:17

passed package-flatpak #240146
flatpak

00:01:48

 
  Test Package
passed autopkgtest-debian-buster-package #240147
librem5

00:02:45

passed lintian-debian-buster-package #240148
librem5

00:01:04