Disable docker cache with github actions

[alexcrichton]

The container in this repository does not take long to build and the caches look like they're take a nontrivial amount of space in CI. Try to free up more room for LLVM caches which are the main reason to use caching in this repository.

[abrown]

I think this is fine, but wanted to note what it seems to change:

• the Build and test (Linux) step calls docker-build.sh, which builds everything in Docker
• the Build x86_64-linux matrix job calls this step and (unscientifically) was around 21 minutes prior to this change
• this change seems to increase the Build and test (Linux) step to 26 minutes, presumably because we need to re-pull some Docker images (?)

In any case, I don't think this is the longest pole in the tent: the Build x86_64-windows job is taking 40-50 minutes... wonder why that is so much slower?

[alexcrichton]

This should only affect the "Finalize wasi-sdk artifacts" action and shouldn't have any affect on the other jobs. Specifically the "Build and push wasi-sdk docker image" step is affected by this caching. Looking at a previous run it looks like it took 52s and on this PR it took 5m49s. I didn't realize this would lead to such a large discrepancy in performance. My guess is that building the arm64 container image requires a lot to run through QEMU and that probably isn't the fastest thing in the world.

[alexcrichton]

Oh, as for windows, looks like while a cache was restored ccache -s reported mostly misses, so for whatever reason the restored cache wasn't valid (not sure why myself)