CI: Improve build times with caching

[seberg]

@jameslamb after using CUDAARCH="RAPIDS", which I think is OK and probably not much more than all-major anyway. The build jobs on this repo take ~40 minutes. (I think libraft is correctly found and all. I am not certain that the Python package build isn't building a second time though right now.)

Either way, it seems like it may make sense to set up caching for the repo soon to cut down build times.

[seberg]

that the Python package build isn't building a second time

it indeed is building a second time. We should try to clean up the build.sh to prevent this.