[WIP] nixos: make several modules not run as root

[peterhoeg]

Motivation for this change

A number of services are currently run as root because they need to create various directories. We can have systemd do that for us so they can start as regular users.

This isn't tested yet.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[peterhoeg]

I don't think all these should be merged in one go, so I'm closing this, creating a tracking issue and doing individual PRs for each instead.