Speed up end-to-end CI

[echeran]

We can speed up our end-to-end CI in different ways:

• Cache Rust Cargo build artifacts
• Split up executor work per-platform (or per-{platform, version})

[sffc]

It appears that most of the time is being spent compiling code and running tests, as it should, without too much wasted time. It appears that dart_web is especially slow, and collation is slow. We have made a number of improvements such as parallelising the executor schema validation and the verifier, and removing a lot of logging to stdout. However, it is not always clear from stdout where the time is being spent.