flatpak: Pin external dependency versions

Stop tracking master branch for external dependencies, prefer
released/tarball versions (even unstable ones).

#199
3 jobs for tchaik/199-pin-flatpak-deps in 4 minutes and 54 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #390935
flatpak

00:04:23

 
  Deploy
passed #390936
review

00:00:30

manual #390937
allowed to fail manual
stop_review