nixos/users: require one of users.users.name.{isSystemUser,isNormalUser}

As the only consequence of isSystemUser is that if the uid is null then
it's allocated below 500, if a user has uid = something below 500 then
we don't require isSystemUser to be set.

Motivation: https://github.com/NixOS/nixpkgs/issues/112647

nixos/modules/services/backup/borgbackup.nix

@@ -169,6 +169,7 @@ let
         (map (mkAuthorizedKey cfg false) cfg.authorizedKeys
         ++ map (mkAuthorizedKey cfg true) cfg.authorizedKeysAppendOnly);
       useDefaultShell = true;
+      isSystemUser = true;
     };
     groups.${cfg.group} = { };
   };

nixos/modules/services/databases/pgmanage.nix

@@ -197,6 +197,7 @@ in {
         group = pgmanage;
         home  = cfg.sqlRoot;
         createHome = true;
+        isSystemUser = true;
       };
       groups.${pgmanage} = {
         name = pgmanage;

nixos/modules/services/misc/bazarr.nix

@@ -64,6 +64,7 @@ in
 
     users.users = mkIf (cfg.user == "bazarr") {
       bazarr = {
+        isSystemUser = true;
         group = cfg.group;
         home = "/var/lib/${config.systemd.services.bazarr.serviceConfig.StateDirectory}";
       };

nixos/modules/services/misc/nix-daemon.nix

@@ -21,6 +21,7 @@ let
          calls in `libstore/build.cc', don't add any supplementary group
          here except "nixbld".  */
       uid = builtins.add config.ids.uids.nixbld nr;
+      isSystemUser = true;
       group = "nixbld";
       extraGroups = [ "nixbld" ];
     };

nixos/modules/services/monitoring/tuptime.nix

@@ -34,7 +34,10 @@ in {
 
     users = {
       groups._tuptime.members = [ "_tuptime" ];
-      users._tuptime.description = "tuptime database owner";
+      users._tuptime = {
+        isSystemUser = true;
+        description = "tuptime database owner";
+      };
     };
 
     systemd = {

nixos/modules/services/networking/bird.nix

@@ -73,6 +73,7 @@ let
           users.${variant} = {
             description = "BIRD Internet Routing Daemon user";
             group = variant;
+            isSystemUser = true;
           };
           groups.${variant} = {};
         };

nixos/modules/services/networking/ncdns.nix

@@ -243,8 +243,10 @@ in
         xlog.journal = true;
     };
 
-    users.users.ncdns =
-      { description = "ncdns daemon user"; };
+    users.users.ncdns = {
+      isSystemUser = true;
+      description = "ncdns daemon user";
+    };
 
     systemd.services.ncdns = {
       description = "ncdns daemon";

nixos/modules/services/networking/pixiecore.nix

@@ -93,6 +93,7 @@ in
     users.users.pixiecore = {
       description = "Pixiecore daemon user";
       group = "pixiecore";
+      isSystemUser = true;
     };
 
     networking.firewall = mkIf cfg.openFirewall {

nixos/modules/services/networking/pleroma.nix

@@ -75,6 +75,7 @@ in {
         description = "Pleroma user";
         home = cfg.stateDir;
         extraGroups = [ cfg.group ];
+        isSystemUser = true;
       };
       groups."${cfg.group}" = {};
     };

nixos/modules/services/security/privacyidea.nix

@@ -264,6 +264,7 @@ in
 
       users.users.privacyidea = mkIf (cfg.user == "privacyidea") {
         group = cfg.group;
+        isSystemUser = true;
       };
 
       users.groups.privacyidea = mkIf (cfg.group == "privacyidea") {};
@@ -294,6 +295,7 @@ in
 
       users.users.pi-ldap-proxy = mkIf (cfg.ldap-proxy.user == "pi-ldap-proxy") {
         group = cfg.ldap-proxy.group;
+        isSystemUser = true;
       };
 
       users.groups.pi-ldap-proxy = mkIf (cfg.ldap-proxy.group == "pi-ldap-proxy") {};

nixos/modules/services/web-apps/nextcloud.nix

@@ -607,6 +607,7 @@ in {
         home = "${cfg.home}";
         group = "nextcloud";
         createHome = true;
+        isSystemUser = true;
       };
       users.groups.nextcloud.members = [ "nextcloud" config.services.nginx.user ];