CI caching

[janiversen]

Let the python build do the caching.

[alexrudd2]

aaaah, no don't do that! (too late 😆 )

setup-python only caches Python (which is fast), and not pip installs (which is slow). See actions/setup-python#330

Before:

After:

[janiversen]

According to the doc it does cache pip, see the parameters used.

[alexrudd2]

According to the doc it does cache pip, see the parameters used.

Yes, that's what the docs say :) But if you look more closely it only caches the downloads, not the install. So it's quite slow. (see screenshots)

[alexrudd2]

Unfortunately I cannot show you a run from this project, since it got deleted. (Hmm, maybe I can open a "fake" pull request that reverts #1796 )

Here's an example of using syphar/restore-virtualenv@v1, which caches the installed pip packages. See how the restore is very fast, and installing dependencies is totally skipped.

[janiversen]

I am aware that python-checkout caches the cache, but it a step I needed to make, before making a cache mechanism tailored to us.