`github.ref` is now defined for both pull requests and pushes, meaning
that it can be used to simplify the concurrency group.
`cancel-in-progress` is set to true only if the trigger is a pull
request, as we don't want master runs to cancel each other out.
Setting the label `ci:skip-news` will skip the job. This is useful for
maintainers to indicate to contributors that a feature isn't big enough
to warrant a news entry, or for contributors who dislike red CI even if
there's nothing wrong.
Also change label `ci-s390x` to `ci:s390x`; this way it'll be easier to
see that `ci:` are a subcategory of labels that affect CI in some way.
These are meant for expensive or situational tests that may not be
suitable to run each time, but can occasionally be useful.
Currently only add testing for s390x. The job is enabled by adding the
ci-s390x label in github.
