CI: Drop MESON_ARGS, and detect them from the Flatpak manifest

See:
https://mail.gnome.org/archives/desktop-devel-list/2020-April/msg00006.html

Note that, even though the CI had installed tests enabled, they weren't
actually used because the CI doesn't have the infrastructure to run
installed tests.

!133
2 jobs for master in 11 minutes and 9 seconds
Status Job ID Name Coverage
  Test
passed #736239
flatpak
flatpak

00:10:01

 
  Deploy
passed #736240
nightly

00:01:07