feat: cancel stale actions workflows (#741)
feat: cancel stale actions workflows (#741)

Syncs with latest addon blueprint, per https://github.com/ember-cli/ember-cli/pull/9697

This uses Github Action's
[concurrency](https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions#concurrency)
feature to kill the workflow runs of commits that are not the current
head commit of a PR.  `head_ref` is only set for PRs
[source](https://docs.github.com/en/actions/learn-github-actions/contexts#github-context),
which is why we fall back to `ref` (so push workflows will run one job
for each branch for the most recent commit).
simplabs/ember-cookies,simplabs/ember-cookies