Revert "ci: Disable Flatpak generation for now"

This reverts commit bcfe0ad1.
5 jobs from wip/hadess/re-add-flatpak in 14 minutes and 8 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #56044
build

00:10:53

 
  Test
passed #56152
flatpak master

00:03:14

failed #56064
flatpak master

00:06:44

failed #56053
flatpak master

00:00:39

failed #56045
flatpak master

00:00:41