Download, rather than sccache-cache, LLVM in CI

[Mark-Simulacrum]

My hope/expectation is that we can do better than sccache in CI for cached builds -- currently it looks like on macOS those still take upwards of 10-11 minutes, which is a significant amount of time that we could potentially cut.

This enables this mode for all non-dist builders; this should avoid any problems with the artifacts we distribute, while also providing for faster test builders (since they'll make use of PGO'd LLVM on the platforms we do that on, which is hopefully a nice win). It slightly increases the chance of test builders starting to fail only after a PR is merged (if PGO changes runtime behavior), but that should hopefully never happen, so I think this is worthwhile.

Measurements on the PR for apple-1 don't show any noticeable improvement in CI times, but those can be pretty noisy -- I'm inclined to land this since it should pretty much always be better and we can reconsider if that ever turns out not to be the case.