For the discussion that led to this, see #354270.

I think an uncontroversial way forward would be to do the following going forward:

• Base haskell-updates PRs on master for fast testing on Hydra.
• Merge haskell-updates into staging as a general rule. When staging-next is in an early phase we can consider merging it into staging-next on a case per case basis.

Outstanding tasks to make this happen (not sure if this is exhaustive?!):

• Finish haskellPackages: update stackage and hackage #354270 and the following staging-next rotation. Unfortunately, the PR is currently based on staging.
• Update the maintainer documentation, i.e. HACKING.md.
• Update the merge-and-open-pr.sh maintainer script
• Update periodic-merge-24h.yml to merge the merge base of staging and master into haskell-updates to prevent haskell-updates from being ahead of staging. This should be possible with GitHub action's outputs from jobs, but I don't know how it'd precisely work.

cc @NixOS/haskell @vcunat @K900