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.
4 jobs for wip/christopherdavis/dual-install in 21 minutes and 11 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #406379
ubuntu

00:07:24

 
  Test
passed #406380
flatpak

00:12:02

 
  Deploy
passed #406381
review

00:00:49

failed #406382
allowed to fail manual
stop_review

00:00:54

 
Name Stage Failure
failed
stop_review Deploy
 * [new tag]         geary-0.8.0             -> geary-0.8.0
* [new tag] geary-0.8.1 -> geary-0.8.1
* [new tag] geary-0.8.2 -> geary-0.8.2
* [new tag] geary-0.8.3 -> geary-0.8.3
* [new tag] geary-0.9.0 -> geary-0.9.0
* [new tag] geary-0.9.1 -> geary-0.9.1
Checking out bfcee9bf as wip/christopherdavis/dual-install...
fatal: reference is not a tree: bfcee9bff7081a0654b9c86217bc6085dcf6a871
ERROR: Job failed: exit code 1