Speed up CI

[joepio]

The Github actions for testing and code coverage take 13 and 15 minutes, respectively.

The following are quite slow:

# These libraries are needed for tauri, and can be quite big.
sudo apt-get install -y webkit2gtk-4.0 libappindicator3-dev # 1 to 10 minutes
# Code coverage is slow because it uses `--nightly` builds, which isn't cached
cargo llvm-cov --all-features --workspace --lcov --output-path lcov.info