🦃 Use sccache in CI with GHA storage, mac & wasi included

[gpshead]

https://github.com/mozilla/sccache from Mozilla is a modern ccache replacement that uses cloud storage such as GHA directly.

If successful, this adds ccache to places we didn't have it before (macOS), and uses GHA storage directly instead of the old ccache based flow of storing a bundle into the GHA cache after the run.

Q: is that an accurate description of how the old ccache flow actually worked?

Windows? it can! That'll take a little more plumbing into its build flow but it should just work.

TODO items

• look at the dumped cache stats across GHA CI runs on commits below, why are many of them so low when they should've just rebuilt the same thing? what inputs does sccache use as a key? is there a unique path or env var mixing things up? they in theory designed this in part for CI usage so...
• figure out the debugging story for sccache in CI to understand why it is or isn't caching some stuff.
• decide which places this is worth enabling vs the older ccache stuff.
• Q: how do we monitor GHA Cache storage and rate limits for the project?

[eli-schwartz]

If successful, this adds ccache to places we didn't have it before (macOS), and uses GHA storage directly instead of the old ccache based flow of storing a bundle into the GHA cache after the run.

ccache itself should work fine on macOS already (and Windows!) so configuring that directly should be an option too.

look at the dumped cache stats across GHA CI runs on commits below, why are many of them so low when they should've just rebuilt the same thing? what inputs does sccache use as a key? is there a unique path or env var mixing things up? they in theory designed this in part for CI usage so...

I know that sccache does cache on the full path despite that this is fundamentally a pretty bad idea (and renders it totally useless for scenarios such as speeding up Gentoo compilations) so that may very well be the problem.