Key the CI run-keeping on the label the pipeline PRs already carry - #687

Follow-up to #684, which merged with the question still open in its body: whether to key this on a new keep-runs label or on pipeline, which the screenshot pull requests already carry. This takes the second option, so nobody has to remember a second label.

-  cancel-in-progress: ${{ !contains(github.event.pull_request.labels.*.name, 'keep-runs') }}
+  cancel-in-progress: ${{ !contains(github.event.pull_request.labels.*.name, 'pipeline') }}

What this changes in practice

keep-runs has since been created and applied to #661 and #666, so #684 is no longer inert. Moving to pipeline widens who is covered:

That is the point if pipeline means "this pull request exists to exercise CI", since those are exactly the ones whose artifacts are worth keeping. It is the wrong call if pipeline is used more loosely, in which case #684 is already right as it stands and this should be closed.

Worth knowing eitherway: #414, #517 and #519 also carry wontfix, so they are long-lived branches that would now run every push to completion.

Test plan

• YAML.load_file parses the workflow and reads the expression back, so the syntax is sound.
• This pull request carries no labels, so it is the unlabelled case: a second push to it should cancel the first run, as today.
• Labelling any pull request pipeline and pushing twice should leave both runs alive, each uploading its own artifacts.