nsswitch: add systemd module

In order for DynamicUser = true to work in services, we need the
nss-systemd module to be able to resolve the user and group names
generated dynamically.

nixos/modules/config/nsswitch.nix

···
   passwdArray = [ "files" ]
     ++ optional sssd "sss"
     ++ optionals ldap [ "ldap" ]
-    ++ optionals mymachines [ "mymachines" ];
+    ++ optionals mymachines [ "mymachines" ]
+    ++ [ "systemd" ];
 
   shadowArray = [ "files" ]
     ++ optional sssd "sss"