WIP: build: run `crossroad update` before installing each dependency set.

It's a test for myself to improve the CI. Don't merge.

Lately we have a bunch of CI failure because of package not found. I am unsure of the exact problem yet, hence this first attempt of a possible solution. The idea is that maybe the repository listing is already outdated (i.e. between dependencies installation for babl, then GEGL, then GIMP). I am really not sure this is the core issue, but this is a test for now.