[test] Use yarn frozen lockfile

[rosskevin]

• I have followed (at least) the PR section of the contributing guide.

Found the ci config to be a bit out of date on the yarn cache and problematic yarn command which can cause unexpected diffs:

• use --frozen-lockfile to avoid diff between repo and ci runs
• update the yarn cache to that reported by the cli in the Check versions and env step

extracted from #14048 (in case that one needs closed)

[oliviertassinari]

@rosskevin How do you want to move forward?

[oliviertassinari]

v2

[rosskevin]

Given the false errors on the ci diff step we have options:

1. accept this PR as-is because it solves that (preferably with dependency update automation)
2. kill the ci diff step and remove frozen from this PR
3. kill the frozen from this PR and ?? whenever there is a diff problem in the future try to figure it out?

[oliviertassinari]

1. sounds good. I have reverted the cache version. It that OK for you?

[rosskevin]

Why revert the cache version when the CLI is reporting the location as v4?

@eps1lon do we still need the offline workaround? If not we don't need the cache version at all, just take out the offline code and strip off the yarn cache version segment altogether as recommended by https://circleci.com/docs/2.0/yarn/

[rosskevin]

One point of this PR @oliviertassinari was to update the cache and use the directory reported by yarn on ci, I'm not sure why you have removed that change. To me it appears you put it back to using a no-longer used/stale cache dir.

[rosskevin]

[oliviertassinari]

Oh ok. I'm reverting my change.

[rosskevin]

This is good to merge as-is. If we are at the point where we can dump the offline cache we can simplify it further so we don't spec a version at all.