Fix additional occurrence of yarn cache nesting

[joshwlewis]

The current yarn cache restore logic (introduced in #978) can create nested caches like below, iff the .yarn/cache folder is committed in the repo, but empty.

- node
  - cache
    - yarn
      - yarn
        - yarn
          - yarn

This PR changes the cache restore logic to remove the empty folder before moving the cache, so we essentially replace the folder, rather than nesting folders.

This also adds some temporary logic to delete any nested caches we created in #978.

GUS-W-10519616