Test external plugin building in buildbot.

The goal here is to test that the fix from MDEV-40608 stays operational. That is, it builds plugins.

The simplest way of doing it would be a buildbot step similar to install, something like
1. installs only MariaDB-devel (or libmariadb-dev)
2. clones some small (fast to clone) and stable (doesn't change, so any errors are caused by the server changes) plugin
3. builds it.

Ideally it would then also install MariaDB-server, load plugin, and see if it works. Hmm, perhaps this all can simply be the part of the install step?