Make ccache caching work on CI

[zbeekman]

RUN --mount=type=cached,target=... caches are not preserved/used between steps on CI. On my laptop, when the configure and build LLVM layer of the "builder" image in the dockerfile can no longer be pulled directly from the cached layer (say a previous layer was updated or needs to be rebuilt) the build time--thanks to the ccache cache--is cut from ~2 hours to ~4 minutes. However, with GHA CI, these caches seem to not be reused between runs, so a full 2.5 hour rebuild of LLVM takes place if the layer needs to be rebuilt.

[zbeekman]

See https://github.com/Mahoney-playground/docker-cache-action for inspiration