Bump ubuntu image to 22.04
Motivated by https://github.com/actions/runner-images/issues/6399. wasm32 CI expects to use the ubuntu image to build the bindists, which should work out of the box on ubuntu-latest GitHub workflows.
Motivated by https://github.com/actions/runner-images/issues/6399. wasm32 CI expects to use the ubuntu image to build the bindists, which should work out of the box on ubuntu-latest GitHub workflows.