Draft: ci: build pip cache before launch py3 jobs to speed them up

The idea is to pre-build pip cache before launching all the py3 jobs to avoid redownloading all the time all the packages from pip.

I'm not sure this is fully working since we don't have a cache server and I keep getting those:

Checking cache for pip-cache-1-non_protected... No URL provided, cache will not be downloaded from shared cache server. Instead a local version of cache will be extracted.

Also there are no timing information of how much time each step takes so it's hard to know if this really speeds up thing :/

And the CI is pretty unstable to comparing with other jobs doesn't seems to show a huge difference :<

Closes #785