Add `just` to the build dependencies

Add the just tool to the build dependencies.

It is needed to build the cosmic-icons recipe that was added in cookbook@4957549c.

The dockerfile in docker/Docker doesn't build anymore because the https://static.redox-os.org/toolchain/apt/ apt repository has been retired. We still add just because there is no reason to break the dockerfile even more.

Regarding .gitlab-ci.yml I am not sure what to do. My understanding is that the goal is to build the CI images only from binary packages and that we don't need the dependencies of the recipes. (Note that cosmic-icons not in the ci packages at the moment). This MR also adds a check for just in mk/depends.mk, but this means that the make ci-img in this repository's pipeline will fail if just is not installed. I see two options:

1. What this MR proposes: We also add just to .gitlab-ci.yml even if it is not needed to build the CI images.
2. Find some way to check if just is present only if the recipes are going to be built.