Set up meaningful CI

[cpovirk]

I had said this on cbe846f:

I notice that this commit adds some workflow. It would be neat for us to
adapt those to actually build our annotated JDK, thereby catching at
least simple mistakes. Or maybe we should just delete them. For now, I'm
leaving them in place, even though they fail because the files refer to
JSpecify annotations, which don't exist in this repo.

The fix might be as easy as adding our annotations somewhere (along with appropriate changes to module-info).

Whatever we do, clearly we don't need to run on n different platforms.

[cpovirk]

(eisop and typetools do appear to put their annotations into the JDK in some sense during their own CI: https://github.com/typetools/jdk/blob/master/README.md#qualifier-definitions. My efforts in #100 didn't pull that off, but I'd imagine that their lead could be helpful.)