Motivation for this change

This allows overriding the static user ids and group ids.
It fixes the situation where a user module tries to redefine an id. Previously the id would be determined by the module loading order (?), but now it requires lib.mkForce and ambiguities will be detected.
This undetected nondeterminism has caused a regression on our systems by suddenly ignoring an important override.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @