Rethink GitHub Actions

[CGMossa]

In similar vain to https://github.com/extendr/libR-sys/issues/204, I believe we ought to outline the requirements for CI here, and work towards those.

• Maintainers dealing with changes upstream (new version of R, Rtools, usual breakage)
• Contributors that just want to focus on rust-api, namely extendr, and no sys-problems
• ???

Currently, we run tests that generates bindings on libR-sys's main branch. This is set in the repo itself, and not only in the CI script.

I don't think we should do this here at all. I think extendr should rely on the precomputed bindings that libR-sys provides, and that this should be what is expected on this project and what is used.

If we are to have such an involved CI for extendr, I think we should choose to vendor libR-sys or somehow merge them together, because right now, we have intricate CIs in both projects.

Issue

I discovered that extendrtests does not catch the version of libR-sys that is used in extend, meaning the live version of libR-sys was never really tested before.