Issue #3840: Arch added a mypaint-brushes1 package.
This is the base system for our CI, and the former mypaint-brushes package got bumped to v2, which broke the build. Instead of rolling back, they added a mypaint-brushes1 package. Let's rely on it instead of installing it ourselves now.
Creating it as a MR since it changes the CI scripts. So let's test properly if this change does not break the CI.