[Backport release-25.11] nixos/nix-daemon: Support running as non-root user

[artemist]

Cherry-pick of bd49e43 from #491809 with fixes from 3e7afcb from #493417

This is being backported to allow using it in the upstream Nix tests.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

[github-actions]

This report is automatically generated by the PR / Check / cherry-pick CI workflow.

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Please follow the backporting guidelines and cherry-pick with the -x flag.
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Warning

Couldn't locate the cherry-picked commit's hash in the commit message of 173eacf.

Hint: The full diffs are also available in the runner logs with slightly better highlighting.

Intentionally different, since fixes are squashed in . Everything is properly referenced in the commit message.