Save `site-packages` as cache in CircleCI job
# What does this PR do?

Currently, we save `~/.cache/pip` as cache. Take `check_repository_consistency` job as example:
- it installs `[all, quality]`
- loading cache takes ~ 45 seconds
- `pip install` takes ~ 3-4 minutes

If we save `.pyenv/versions` as cache too:
- loading this new extra cache takes ~ 2 min
- `pip install` takes  20 ~ 30 seconds

We gain 30 ~ 90 seconds (depending on CircleCI's states). Not a big absolute improvement. But for this job which total runtime is ~ `5m30s`, we can say > 20% reduction. As `check_repository_consistency` and `check_code_quality` will be run for each PR's each push, probably it's nice to have such reduction.

WDYT?




_The documentation is not available anymore as the PR was closed or merged._<|||||>>If we save .pyenv/versions as cache too:

>loading this new extra cache takes ~ 2 min
pip install takes 20 ~ 30 seconds


@ydshieh Wait - are the timings of these in the right order? <|||||>> Do we still get updates if there is a release of one of the libraries?

@sgugger Yes if we use -U flag. I can do that if you are ok<|||||>> > If we save .pyenv/versions as cache too:
> 
> > loading this new extra cache takes ~ 2 min
> > pip install takes 20 ~ 30 seconds
> 
> @ydshieh Wait - are the timings of these in the right order?

Hmm, yes. But could you explain why you have doubts so I can reply in more details?<|||||>> @sgugger Yes if we use -U flag. I can do that if you are ok
YEs, please!<|||||>> Hmm, yes. But could you explain why you have doubts so I can reply in more details?

@ydshieh I just realised my mistake 🙃  I thought it was saying that it takes 2 mins to load with the cache and 30-40s to install by pip. Whereas it's (45 secs + 3-4 mins ) -> (2 mins + 20-30s). My bad! <|||||>> > Hmm, yes. But could you explain why you have doubts so I can reply in more details?
> 
> @ydshieh I just realised my mistake 🙃 I thought it was saying that it takes 2 mins to load with the cache and 30-40s to install by pip. Whereas it's (45 secs + 3-4 mins ) -> (2 mins + 20-30s). My bad!


The new one should be (45 secs + 2 mins + 20-30s): The first part of cache (in `.cache/pip`) is not changed.
But we still have a little gain overall.

<|||||>Although already been approved - FYI: I just added -U everywhere