Improve the CI runtime

[Systemcluster]

Follow-up to #4051, see there for prior discussion.

This splits up the longest-running CI jobs into smaller jobs to improve the CI runtime.

The overall usage didn't go down much, it's still around 100 minutes, but now it's spread out over more jobs.
(It's gotten a bit more stable slightly under the 100 minute mark with this PR, but not much.)

Additional changes are commented below.

[daxpedda]

I'm happy to give it a final review already as-is, up to you when its ready!

I believe the last three things we could do are:

• Parallelize build_examples via bash somehow (or if you know your way around webpack maybe there is something there?).
• Remove the test --all-features -p web-sys, it can be tested without all features, but we have to add the required features to dev-dependencies. Checking building with all features is already done in another part of the CI.
• Cross-compile dist_windows from Linux, which should finally get us down to under 3 minutes (if the other two points work out!).

Could all be done in a follow-up btw.

[Systemcluster]

I didn't get it under 6 minutes, but the later commits touch up some details that might still be worthwile.

I'm done with the CI for a while, feel free to modify and force push this branch if something should change. I'll leave any additional improvements for you or someone else (or for me at some later time 🙂).

[Systemcluster]

It looks like it didn't work, maybe it's not (just) here after all? I'm pretty sure it's always the last test, so maybe the waiting needs to happen at a higher level (before shutting down tokio runtime?).

[daxpedda]

"Run wasm-bindgen crate tests with various environment variables" could still be split up, that ones the longest now.

I'm happy to merge it as-is if we can split out the caching.

[daxpedda]

Thanks for all the work!

[Systemcluster]

Parallelize build_examples via bash somehow (or if you know your way around webpack maybe there is something there?).

I know that there are many faster webpack alternatives (rollup, rspack or parcel as bundlers, or maybe bare swc or esbuild can do it as well) that might be easy to switch to, I haven't looked at the webpack setup in detail to be able to tell.

I assume just running the loop in parallel is a bit easier though. parallel or something like this would surely work.

[Systemcluster]

I need to add (with this as a follow-up, and it only showing in linked CI runs) that adding sccache really doesn't seem to help much with the wasm-bindgen CI. The jobs it speeds up aren't a bottleneck, and it doesn't seem to speed up the slow jobs for various reasons. It also doesn't reduce the 100 minute usage much.
@daxpedda's comment in the closed PR is on point, other gains should be prioritized, unless I missed some configuration that would improve sccache here.