Compiling Rust dependencies with cargo is slow

[LightAndLight]

Example job: https://github.com/LightAndLight/ipso/actions/runs/3505790488/jobs/5872341175 (click the settings icon and enable timestamps). Total duration: 4.5 minutes.

Compiling Rust dependencies for cargo test takes ~30 seconds for definitely-non-ipso-packages (start, end), and ~60 seconds for all Rust packages including ipso packages (start, end). That's 10-20% of the job duration spent on setting up dependencies, most of which don't change between runs.

Try caching the target directory (will it be too big?)