Handle initialPassword and initialHashedPassword for !mutableUsers

In this case, they're equivalent to setting ‘password’ and
‘hashedPassword’ (since there is no distinction between an initial and
non-initial user account state).

nixos/modules/config/users-groups.nix

@@ -235,6 +235,14 @@ let
           useDefaultShell = mkDefault true;
           isSystemUser = mkDefault false;
         })
+        # If !mutableUsers, setting ‘initialPassword’ is equivalent to
+        # setting ‘password’ (and similarly for hashed passwords).
+        (mkIf (!cfg.mutableUsers && config.initialPassword != null) {
+          password = mkDefault config.initialPassword;
+        })
+        (mkIf (!cfg.mutableUsers && config.initialHashedPassword != null) {
+          hashedPassword = mkDefault config.initialHashedPassword;
+        })
       ];
 
   };