home-environment: use per-user profile path in /etc

Before the profile directory value would point directly to the build
output in the Nix store. Unfortunately this would cause an infinite
loop if the user's configuration directly or indirectly refers to the
profile directory value.

Fixes #1188

nixos/default.nix

@@ -88,6 +88,8 @@ in {
       (mapAttrs (username: usercfg: { packages = [ usercfg.home.path ]; })
         cfg.users);
 
+    environment.pathsToLink = mkIf cfg.useUserPackages [ "/etc/profile.d" ];
+
     systemd.services = mapAttrs' (_: usercfg:
       let username = usercfg.home.username;
       in nameValuePair ("home-manager-${utils.escapeSystemdPath username}") {