nixos: pass driver version to user service

The user-service drop-in invoked activate without a driver version, so
the script defaulted to driver 0 and always managed the per-user
home-manager profile, ignoring enableLegacyProfileManagement. Use the
same version as the system service and assert both generated
user-service commands.

nixos/default.nix

@@ -9,6 +9,7 @@
 let
   inherit (lib) mkIf;
   cfg = config.home-manager;
+  driverVersion = if cfg.enableLegacyProfileManagement then "0" else "1";
 
   baseService = username: {
     Type = "oneshot";
@@ -75,7 +76,6 @@ in
         _: usercfg:
         let
           inherit (usercfg.home) username homeDirectory activationPackage;
-          driverVersion = if cfg.enableLegacyProfileManagement then "0" else "1";
         in
         lib.nameValuePair "home-manager-${utils.escapeSystemdPath username}" (
           lib.attrsets.recursiveUpdate (baseUnit username) {
@@ -145,7 +145,7 @@ in
           packages = [
             (pkgs.writeTextDir "${hmDropIn}/10-user-activation.conf" ''
               [Service]
-              ExecStart=${home.activationPackage}/activate
+              ExecStart=${home.activationPackage}/activate --driver-version ${driverVersion}
             '')
           ];
         }

tests/integration/nixos/legacy-profile-management.nix

@@ -50,7 +50,29 @@
     let
       legacy = "${nodes.machine.system.build.toplevel}/specialisation/legacy";
       modern = "${nodes.machine.system.build.toplevel}/specialisation/modern";
+      userServiceDropIn =
+        enableLegacyProfileManagement:
+        let
+          nixosEval = import "${pkgs.path}/nixos/lib/eval-config.nix" {
+            system = pkgs.stdenv.hostPlatform.system;
+            modules = [
+              ../../../nixos
+              {
+                system.stateVersion = "24.11";
+                users.users.alice.isNormalUser = true;
+                home-manager = {
+                  startAsUserService = true;
+                  inherit enableLegacyProfileManagement;
+                  users.alice.home.stateVersion = "24.11";
+                };
+              }
+            ];
+          };
+        in
+        (builtins.head nixosEval.config.users.users.alice.packages).text;
     in
+    assert pkgs.lib.hasInfix " --driver-version 0\n" (userServiceDropIn true);
+    assert pkgs.lib.hasInfix " --driver-version 1\n" (userServiceDropIn false);
     ''
       start_all()