Replace yarn cache rather than nest it

[joshwlewis]

It looks like the current yarn cache save logic (introduced in #978) creates nested caches like this:

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

This PR changes the cache save logic to replace the existing yarn folder in the cache, rather than creating a directory inside.

GUS-W-10519616

[edmorley]

This will prevent new cases from occurring, but will existing nested caches need additional clean-up (either here or in a follow-up PR)?

[joshwlewis]

This will prevent new cases from occurring, but will existing nested caches need additional clean-up (either here or in a follow-up PR)?

This should clean up old caches (by virtue of yarn being pretty good at cleaning up it's own cache).

[joshwlewis]

@edmorley -- You were right, this does not clean up nested caches. I added clean up logic here: 2ec5589.