gitlab-ci: don't use mypaint-brushes from Archlinux.
If I get it correctly, archlinux bumped the mypaint-brushes package to v2, which broke our CI. The v1 and v2 brushes are not the same, and they are not even compatible (GIMP only supports v1 brushes so far). These should be different packages, hence the incremented major versionning. For the archlinux change, see also: https://git.archlinux.org/svntogit/packages.git/commit/trunk?h=packages/mypaint-brushes&id=dfd032b3aab40a46c751bd47713fb82bf11bd984 Let's just install the brushes ourselves from the correct branch. With this kind of changes, as well as the weird patches they do when they rename the pkg-config file of libmypaint, I think we should consider not rely on this distribution for our CI.