Description:

When installing the `nix` arch package, nix commands can be executed as root, but fail as unprivileged users.

$ nix-shell -p hello

warning: Nix search path entry '/nix/var/nix/profiles/per-user/twix/channels/nixpkgs' does not exist, ignoring
warning: Nix search path entry '/nix/var/nix/profiles/per-user/twix/channels' does not exist, ignoring
error:
… <borked>
at «none»:0: (source not available)

… while calling the 'import' builtin
at «string»:1:18:

1| {...}@args: with import <nixpkgs> args; (pkgs.runCommandCC or pkgs.runCommand) "shell" { buildInputs = [ (hello) ]; } ""
| ^

(stack trace truncated; use '--show-trace' to show the full trace)
error: file 'nixpkgs' was not found in the Nix search path (add it using $NIX_PATH or -I)
at «none»:0: (source not available)

The upstream multi-user installation script [1] seem to create only the nix-daemon socket [2] and no other tmpfile, or environment variables.
By keeping only the nix-daemon tmpfile.d script, and removing everything else in tmpfiles.conf, user.environment.conf, user.tmpfiles.conf from the arch package, both root and unprivileged nix commands works.

Is there a reason to have those tmpfiles and environment variables as they are ?

PS: I'm pretty new to Nix, so please take this with a pinch of salt.

Additional info:
* package version(s) : nix 2.16.1-1
* see also :  FS#73120 


Steps to reproduce:
1. # pacman -S nix && systemctl enable --now nix-daemon
2. Apply tmpfiles and environment variables, or reboot
3. # nix-channel --add https://nixos.org/channels/nixpkgs-unstable && nix-channel --update
4. # nix-shell -p hello # Works fine as root
5. $ nix-shell -p hello # Fails as an user, see output above

[1] https://github.com/NixOS/nix/blob/master/scripts/install-systemd-multi-user.sh
[2] https://github.com/NixOS/nix/blob/master/misc/systemd/nix-daemon.conf.in