Only cache `yarn`'s system cache.
Only cache `yarn`'s system cache.

Without this change, `node_modules` will be cached between builds
with different contents (somewhat as a "luck of the draw") due to
the usage of `--skip-cleanup`.

jrjohnson/liquid-fire,ember-animation/liquid-fire,ember-animation/liquid-fire,ef4/liquid-fire,jrjohnson/liquid-fire,ef4/liquid-fire