This PR does a couple of things to fix our github actions/CI process:
Pins the Rust version in Github Actions, because otherwise we get unexpected, new clippy errors when Rust stable has been upgraded to a newer version. Note that this means we'll need to manually upgrade the Rust version in github actions from now on.
Fixes an attribute problem that clippy didn't like.
This PR does a couple of things to fix our github actions/CI process: