ci: simplify workflow

[davidhewitt]

This extracts just the ci workflow file changes from #1457.

Instead of having separate build & test stages, this now just runs tests for various feature combinations. On pypy this command is overridden to just build --lib --tests.

[kngwyu]

Thanks, but ... 🤔, is it good to use environmental variables to shorten commands?
I'm not familiar with CI good practices, though.

[davidhewitt]

I've pushed a commit to use "outputs" of a github actions step, maybe you prefer that?

I wanted to use variables of some form because it simplifies maintenance of the CI scripts - every time we add a new feature, there were a lot of places to edit. Also the various cargo commands were getting inconsistent with PyPy. Now the structure is more consistent on all CI runs, and there's only one place to edit if we change a new feature.

[kngwyu]

I've pushed a commit to use "outputs" of a github actions step, maybe you prefer that?

I'm sorry but I'm thinking that Github actions might provide something better than env...
But it looks like that env is what we want for all-additive-features: https://docs.github.com/en/actions/reference/environment-variables

[davidhewitt]

Ah ok, I'll revert that last commit? Always good to try things 😄

[kngwyu]

I noticed that we can use with instead of env: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#jobsjob_idstepswith, but... I really don't know which is better. They look the same 😕

[kngwyu]

Isn't it better to abort the build with some warnings?

[kngwyu]

I think we can only refactor all-additive-features and leave the PyPy conditioning as is. Since we cannot build the crate with PyPy and abi3, I don't see any strong motivation to commonize them.

[davidhewitt]

Ok will push a commit to rework this PR later today. Thanks for looking at this with me 👍

[davidhewitt]

I'm gonna merge this so that I can rebase #1457 on it. Hope that's alright.

(If there are changes you want, I'm happy to do them in a follow-up PR.)