CI: Cache cargo build artifacts when building mir-json

[RyanGlScott]

The crux-mir CI job takes about 7 minutes to build mir-json on each run, as it must build all of its cargo dependencies from scratch. This is an especially lengthy part of the CI, and it's once that we could conceivably cut down on by caching the cargo dependencies in GitHub Actions. The https://github.com/Swatinem/rust-cache action looks promising for this, as we are using it to great effect in mir-json's CI in this PR.