Each deployment causes go-test and go-check to run, spawning a lot of jobs across all our repos. We quickly build up a queue, due to GitHub Action's usage limits: https://docs.github.com/en/actions/reference/usage-limits-billing-and-administration, and jobs take a long time to execute.
Even worse, when a developer opens a new PR, it will now also sit in the queue.

@mvdan pointed out that GitHub Actions has a concurrency option now:
https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#concurrency.
We could use this option to make sure that only one copy workflow runs at the same time. Iff that workflow opens a new PR, it would sleep for a minute after that.

We could even allow N copy workflows to run concurrently, by setting concurrency: ci-${{ math.Ceil(id/N) }}. This is to speed up the common case of adding new a repository to the workflow: In this case, we run all O(100) copy workflows, but almost all of them won't open a PR. Booting the VM and cloning the repo might still take 15-20s, so running a few jobs concurrently will speed up things.