Issue description

On a NixOS system with mutableUsers = true, the admin is supposed to be able to imperatively add users by running useradd.

The useradd command automatically allocates a UID and GID range for the new user if /etc/subuid or /etc/subgid exist, which they always do on NixOS.

However, all changes to these two files are overwritten on the next system reboot or nixos-rebuild, which to me seems counter to the intent of mutableUsers.

Steps to reproduce

useradd -m testuser
grep '^testuser:' /etc/sub[ug]id # should match
/run/current-system/activate
grep '^testuser:' /etc/sub[ug]id # nothing

Technical details

• system: "x86_64-linux"
• host os: Linux 5.1.14, NixOS, 19.09.git.6cc7e33 (Loris)
• sandbox: yes
• version: nix-env (Nix) 2.2.2

The channel info isn't meaningful because I built from a git checkout, not my normal channels, and I tested in a container that didn't have access to /nix/var anyway.