ci: move some variables closer to their usage These were defined globaly because we had trouble making them inherited. This is now fixed, so we can get them were they belong.

ci: use extends instead of <<: *x The old form is a yaml construct that make it hard to share variable definition. The "extends:" key is a gitlab specific that preserve the variable definition and just add the new ones. This will help us to reduce duplication. This has the effect of fixing some of variants definition we though we set while we did actually not. Most notably, the "rust" variant for 3.12 and 3.13 seems fully broken in the CI (possibly because some rust-cpython version issue?). This changeset only reveal such breackage and does not introduce them.

ci: use the macos wheel to run tests