chore(developer): always clone keyboards repo for tests

During the configure phase for kmcmplib, we'll now always make a clone
of the keyboards repo so that we can checkout the specific commit we
want without making a mess of the user's environment. This is a little
slower (about 30 seconds to do a full local clone, a couple of minutes
for a remote clone) but a whole lot safer.

This only needs to be done in the `configure` action.

This also simplifies our scripts because we no longer need to pass any
option into meson for the keyboards repo, as we'll always know it is in
tests/keyboards.