Update CI for 0.13 branch

Since this branch will only be used for the Ubuntu PPA going forward,
target Bionic and don't bother with the Fedora or Flatpak builds.
2 jobs for <span class="ref-name">wip/0.13-ci</span> in 18 minutes and 12 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #258394
ubuntu

00:09:55

 
  Test
passed #258395
deb-package

00:08:16