There's a trick to train builders without actually starting a building project that someone taught me recently.

The leaves, (and I presume the rubble,) give you skill the moment they succeed in carrying a leaf away. However, it only advances the task when they carry the leaf away and drop it. If you keep interrupting the task to have them carry leaves again, they will NOT advance the task to 1%, and therefore they will not automatically undertake the task, yet they'll still gain skill.

This can help train builders, without having them automatically complete the task that you're using to train builders with.