build: Set up dual install for flatpak

Part of GNOME/Initiatives#1;
Sets up dual installation for flatpaks, allowing us to
build and run geary without affecting our system install.
This will allow testing of nightlies and CI artifacts as well
via the flatpak, so we can iterate on designs and share
bundles to test with.
5 jobs for wip/christopherdavis/dual-install in 18 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #363158
fedora

00:07:39

passed #363159
ubuntu

00:06:51

 
  Test
passed #363160
flatpak

00:10:20

 
  Deploy
passed #363161
review

00:00:41

manual #363162
allowed to fail manual
stop_review