CI: Upgrade cache GH Action & remove fallback caches

[phated]

Issue: N/A

What I did

I'm noticing a bunch of failures on the "Unit Tests" GH Action. I believe this is because the cache action is fetching stale caches. I removed the "restore-keys" option so it only ever pulls a cache that matches the yarn.lock file.

How to test

• Is this testable with Jest or Chromatic screenshots? ❌
• Does this need a new example in the kitchen sink apps? ❌
• Does this need an update to the documentation? ❌

If your answer is yes to any of these, please make sure to include it in your PR.

[gaetanmaisse]

I would go one step further and completely remove the unit tests workflow from the GitHub Actions.
I guess it was added to give it a try with GH Actions but, at the moment, I don't see any benefit to keep it as it is a pure duplicate of what we have on CircleCI, it takes so much time to run and is often failing. What's your opinion on that @ndelangen ?

[phated]

@gaetanmaisse Can we merge this and make that decision later? It is causing people's PRs to be stalled.