Add CI caching for faster builds

[erikmannerfelt]

With #26, the CI build time is now around 13 minutes. That's not extreme, but it could surely be better. Most of the time is most likely spent on compiling the dependencies, and not rsgpr itself. Therefore, assuming the dependency versions don't change, there's no good reason to recompile them every time a new action is triggered.

There are apparently good caching solutions out there. One blog showcases this nicely:
Optimizing Rust Builds for Faster GitHub Actions Pipelines