Disable Flatpak build on public GitLab repo for now

It has been timing out because of the slow shared runner VMs.
Also bump the build to use Fedora 30's container image when
it is being executed.
2 jobs for master-build in 67 minutes and 48 seconds
Status Job ID Name Coverage
  Build
passed #207472448
linux
build_windows

01:00:29

 
  Build Gui
passed #207472449
windows msbuild
build_windows_gui

00:07:18