nixos/systemd: support adding and overriding tmpfiles.d via environment.etc

[orivej]

Motivation for this change

This allows the user to configure systemd tmpfiles.d via environment.etc."tmpfiles.d/X.conf".text = "...", which after #93073 causes permission denied (with new X.conf):

ln: failed to create symbolic link '/nix/store/...-etc/etc/tmpfiles.d/X.conf': Permission denied
builder for '/nix/store/...-etc.drv' failed with exit code 1

or collision between environment.etc and systemd-default-tmpfiles packages (with existing X.conf, such as tmp.conf):

duplicate entry tmpfiles.d/tmp.conf -> /nix/store/...-etc-tmp.conf
mismatched duplicate entry /nix/store/...-systemd-246/example/tmpfiles.d/tmp.conf <-> /nix/store/...-etc-tmp.conf
builder for '/nix/store/...-etc.drv' failed with exit code 1

Fixes #96755

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 nixpkgs-review --run "nixpkgs-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.

/cc @flokli @primeos