Update CI

[jsdw]

I think that we should:

• Run cargo clippy and rustfmt on a standard toolchain (default target, not nightly); I've noticed intermittent issues running those as they are on nightly wasm.
• Have a separate wasm32 test job using that target
• Use the aarch64-unknown-none target for the no_std test (this seemed to work reasonably well in scale-bits at least) to ensure no std paths creep in.
• Use stable checks rather than nightly so that things like rustfmt and clippy don't run the risk of breaking or changing day by day.