CI: Fix poetry caching

[alexdewar]

I followed @AdrianDAlessandro's suggestion in the issue and changed to installing poetry via pipx before the setup-python step.

Fixes #103.

[dalonsoa]

And this fixes the caching issue... why?

[alexdewar]

@dalonsoa The problem is that seemingly you need poetry installed before the setup-python action runs, else the caching doesn't work. And the poetry action we were using doesn't work for this.... Is that right, @AdrianDAlessandro?

[AdrianDAlessandro]

@dalonsoa The problem is that seemingly you need poetry installed before the setup-python action runs, else the caching doesn't work. And the poetry action we were using doesn't work for this.... Is that right, @AdrianDAlessandro?

Yes, that is also my understanding. But it comes with a risk of a semi-hidden bug.

I noticed that if the python version installed by setup-python doesn't match the version in pyproject.toml, it will fall back on trying to use the default/system python version in the selected OS. Which goes one of two ways:

1. Succeeds and runs if that version is compatible with pyproject.toml, but will show something like the workflow "test(ubuntu-latest, 3.12)" succeeded when it actually used Python 3.10
2. Fails if that version is also incompatible with pyproject.toml

Number 2. above is fine, but number 1. is a problem and I'm not sure how to force the workflow to fail in that case (which I think it should)

[alexdewar]

@AdrianDAlessandro Ugh that's kind of annoying. One nasty hack we could do is to add a check for problem 1 on the CI somehow.