Dist directory should be cleaned as part of build process

[tchaffee]

Describe the bug

When the yarn build target is run it does not clean the dist directory. This means that if you delete a challenge seed dir it will still remain in the dist directory.

To Reproduce

Steps to reproduce the behavior:

1. Go to repo base dir.
2. Run yarn build.
3. Delete one of the directories from ./challenges, for example the 03-front-end-libraries dir.
4. Run yarn build again.
5. The 03-front-end-libraries dir will still be in the dist dir.

Expected behavior

Deleted directories should not remain in the dist dir after running yarn build.