ci: use ccache to cache compiler output

[cebtenzzre]

This PR adds ccache to the CI scripts, leveraging CircleCI's save_cache/restore_cache to persist the cache between runs. Leaving as draft until #2941 is merged.

Effectiveness

Without cache:

With hot cache:

The Windows build takes about 50% less time, and all builds complete in around 30 minutes instead of over an hour.

Qt Cache

We are currently using a few gigabytes of cache for Qt, which is OK when the cache hits:

But when it misses, as it so often does, there is a +119% slowdown as it compresses and uploads Qt:

Since ccache is capped to 500MiB and is much more important, I've chosen to remove the Qt cache so it doesn't cause the ccache cache to be evicted.

We may be able to re-enable the Qt cache in the future if we think it's useful and we are able to store ~/.ccache somewhere else, e.g. using sccache and an S3/R2 backing store.

Limitations

Cross-job ccache is suboptimal right now because some jobs use different working directories, and the relative paths to header files matter.
edit: I did some local testing, and it seems like most files get a preprocessor-mode cache hit regardless of build directory. It's not as efficient as a direct hit, but direct hits are difficult to achieve for a project when it is top-level in one case and not in another.