Changes to the flatpak CI templates

Dear GNOME developers,

we have made some changes to the flatpak CI templates to make them use
them use the meson args from the manifest. For most apps, this doesn't
require any changes: you should just drop MESON_ARGS from your
.gitlab-ci.yml.

For apps that modify the template, you should take a look at the
latest version [1] for inspiration. Apps that pass different
MESON_ARGS in different jobs should use probably use different
manifests.

And of course if anything breaks or if you need help, feel free to
ping me on IRC.

Cheers,

Abderrahim

[1] https://gitlab.gnome.org/GNOME/citemplates/blob/master/flatpak/flatpak_ci_initiative.yml