You can't update a GHA cache once you create it, so if we use the same cache
for each job in a matrix build, then we end up populating it for the first job
that completes, so any slower jobs don't get their dependencies cached.
On the other hand, if we create 20 500MB cache items on each build, we're going
to exhaust the cache storage as soon as we do a build. So, instead, let's just
do the caching for the main branch, and hope that other branches can still
benefit from it.
* 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.