CI Tuning

[CleanCut]

Prompted by #1309 (comment)

• Simplified syntax & linted. Think "rustfmt" for github actions toml files.
• Combined apt updates and installations into a single step (why update twice? why separate installations?)
• Future-proofed OS selections by using *-latest, which never gets old, as opposed to specific version numbers.
• Remove cache step after verifying cache isn't being used and is just wasting time.
• 😄 Merge clean job into the linux stable job...which is what I actually started out to do.
• ⚠️ Enable actually building and running tests on Windows and macOS. Apparently they haven't been running for quite some time, though cargo check was.

Before: 8 jobs with typical total runtime of 17m - 18m (CI jobs are very noisy, so total runtime varies quite a bit)
Now: 7 jobs with typical total runtime of 14m. We shaved a few minutes off, and 2 of the remaining builds are actually building and testing now when they weren't before.