CI: Make sure release runs only once

Since we're using `effekt-updater`, the `release` CI runs twice (see [here](https://github.com/effekt-lang/effekt/actions/workflows/deploy.yml)):

We should employ concurrency groups to prevent it from running twice on the same commit (if possible): https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/control-the-concurrency-of-workflows-and-jobs