Our CI is too expensive

Due to the way we run CI and how the repository is set up I'm running out of the free CI budget fast.

This is because

• we require Fast-Forward Merges, which puts a constant rebase churn onto contributors, which keeps retriggering CI
• we use hydra-cli to block until hydra.nix-community.org has finished building all builds, which is not expensive in compute, but expensive in minutes, if our jobs are not ran swiftly.

I had two CI jobs time out on me after an hour, which deductes 120 min from the 400 min free budget. That's a whopping 30%!

And while the average run only takes ~6 minutes, even successful runs can clock in at just under an hour:

What is immediately actionable is to allow merge commits. This will make the Git history slightly more noisy, but relief committers of constantly having to rebase MRs so that they stay mergable.