* Replace custom cancellation action with `concurrency`
* Improve step names
... so don't have three steps with the same name
* Bump version of checkout action
checkout@v2 uses an old version of nodejs, which is deprecated.
Currently, the cache of the xtask binary isn't working terribly well:
* we seem to build it on each run anyway, presumably because we don't cache any of the intermediate build
artifacts. Running the binary directly rather than indirecting via "cargo" prevents this.
* There is no sharing of the cache between the "rust" and "bindings" CI, because we use different cache keys.
This PR addresses both problems, and hopefully speeds up CI a bit as a result.